Most people with an average sex drive and any moral decency are going to be turned off, repulsed, and grossed out by someone randomly and without warning passionately kissing them.
In fact, only someone with raging hormones and a possible need to be arrested by the police, is going to be aroused by this situation you have described.
Sexual assult, even by a friend, is still sexual assult.
What you have described is a sexual assault scene.
It's normal for a person to not become sexually aroused while being sexually assaulted.
Your character's lack of response is normal and does not indicate she is asexual, rather it indicates her attacker to be a sexual pervert and probably also a rapist, considering she sees no reason to ask for permission before touching another person.
While it could be that your character is asexual, her lack of a sexual response while being sexually assaulted by someone she trusted, is not a good indicator scene to tell us she is in fact asexual.
This type of assault is commonly seen in possessive people suffering from Narcissistic Personality Disorder (a type of Sociopathy). They are driven by ego and feel no real love for anyone, using people only to serve a selfish motive. Your character abruptly kissing another character, without permission, for the purpose of finding out if it sexualy arouses her, tells the reader that she is a dangerous, self centered person who uses others and does not care how they feel. This woman screams: VILLAIN.
Also, I'm not sure if you even know what polyamorous means as you seem to be using it incorrectly.
An asexual person has no sex drive and does not want to fix it, because they are not attracted to others. A lack of sex drive is caused by hormone imbalance. Someone who is NOT asexual, lacks sex drive, but is attracted to others and will seek out hormone therapy to curse their lack of sex drive.
There are different types of asexual, depending on how the individual feels emotionally towards other people.
There are asexual people who are straight, who are gay, who are lesbian, who are bisexual, and who are pansexual...meaning they are emotionally attracted to a certain gender type.
An asexual person can be any of the above, but have the added aspect that they do not feel sexual arousal and feels no interest in engaging in sexual activities regardless of the gender they are attracted to.
From what you have said, it sounds to me like your character is either demisexual or aromantic.
Depending on how you want the story to progress, you could have your character be either demisexual or aromantic. Both are types of asexual, but have some very stark differences.
A demisexual is an asexual person who feels sexual satisfaction through emotional instead of physical bonds.
They are the opposite of an aromantic person, who is an asexual who feels not emotional or physical bond.
A demisexual has a deep need to love and feel loved.
An aromantic feels love is a useless emotion and a pointless waste of time.
A demisexual has a deep need for a partner to love them.
An aromantic is often seen by others as "robotic" or "having no emotions" because they are usually physically/medically incapable of feeling love for another person. They may or may not want to love someone, but the synapsis in that part of their brain are not firing, so they simply feel not love towards anyone.
A demisexual feels a strong need to marry - to the point that of any sexual orientation, they are the most likely to practice polygamy and have multiple spouses.
An aromantic feels a deep aversion to marriage, often viewing it as a institution meant to trap and enslave them. Most aromantics with never marry.
Of the two a demisexual is more likely to eventually lose their virginity, while most aromantics will remain virgins their entire lives.
Most demisexuals view sex as a way to show their partner they love them, and will engage in sex, often ONLY if their partner is hurt, injured, sad, or otherwise upset, using sex as a way to cheer up their partner and say "See, it's okay, I'm here for you."
Most aromantics view sex as icky, sticky, messy, gross, and repulsive and will avoid it at all costs.
A demisexual needs to feel they are in a long term, stable relationship before they will engage in sex. Often they date their partners a decade or more before getting married and will only engage is sex after marriage, and even than only if their partner initiates it.
Neither demisexuals or aromantics feel strong sexual attraction to anyone, male or female, and both will be turned off by someone kissing them, no matter what gender they find attractive.
Both demisexuals and aromantics usually feel content to remain virgins their entire lives.
Demisexuals have a tendency to be very social and have a strong need to form close bonds with a romantic partner.
Aromantics have a tendency to be loners, not feeling any need to have a partner, companion, or spouse.
...After all of that... every person is different, so no two demisexual people or aromantic people are exactly alike. Thus they may be exactly like this list or nothing like this list, or they may have some parts of the aromantic list and some parts of the demisexual list at the same time.
Keep in mind that this list is just a generalization to help you pinpoint where your character lays, and is not a one size fits all checklist.
*I am demisexual, btw. I have been with my partner since August 1987. He is asexual. While sex is a part of our relationship, it is rare, and the longest we've gone between sex was 13 years. However we have a strong intellectual and emotional bond. Both of us are people who are highly attracted to intelligent people (whereas the average person is attracted to physical beauty). While the average person gains deep satisfaction from physical touch (sex) we gain deep satisfaction from lengthy debates about various scholarly type topics, from religion to politics to current events to to the latest medical news,. People who have seen us interact, often have the first impression that we are fighting or arguing angrily. We do get quite into our discussions often screaming and yelling at each other. But if you were to pay close attention, you also see that neither of us is every angry, both of us are laughing between the yelling, and the entire engagement is intermingles with us holding hands and hugging, all the while. People often also say that of any couple they know, we are the closest, that we seem to be soul mate and have a close knit bond that defies logic.
We are rarely seen in public without the other, and people are often quick to "call us out" for public displays of affection (meaning walking while holding hands and hugging each other - both of which being things most people consider offensive, but I do not know why.) There is no kissing or sex between us, because find these things to be dull and boring and see them as things that take away from time we could be spending interacting (debating?) together.
And as to the scene you described... if some woman walked up to me and out of the blue kissed me, I'd punch her in the face as an automatic reflex response, before she got close enough to kiss me.
I am someone who has a deep need for my personal space. I have Autism (ACTUAL Autism aka Kanner's Syndrome NOT Aspergers) and keep a 3 foot invisible barrier around me at all times. My partner is the only person ever allowed to step over that line. Anyone else steps that close to me and it triggers an instant anxiety attack meltdown. I will kick them, punch them, bite them, and pull out a machete, as the machete has a 28" blade and is the fastest way to get people to step back out of my space. (Yes, I do carry a machete on me.) I do not take invasions of my personal space lightly, my brain sees any one stepping close to me as an immediate attack and my brain starts going on the defensive.
Someone trying to kiss me in the way your scene describes, would be seen by me as an attempted attack. So regardless of any gender I liked, that situation would never be seen as romantic or something to cause sexual arousal.
In the Quaraun series, you frequently see scenes exactly like the one you have described, in which Unicorn is making unwanted sexual advances towards Quaraun.
Unicorn has a sex addiction problem. He is also a serial rapist. He frequently sexualy abuses Quaraun and has raped Quaraun multiple times.
Quaraun is asexual and is perfectly content to live his life a virgin. You see a point in the series where Quaraun comes to the realization that Unicorn stops raping random strangers when he's with Quaraun, which is the reason Quaraun eventually submits to being Unicorn's lover.
Unicorn has a very obsessive personality and when he finds someone who can tolerate his often childish behaviour, he becomes obsessed with them, to the point of ignoring all others. In being obsessed with Quaraun, he leaves others alone and is no longer rampantly raping everyone who crosses his path. And it is for this reason which Quaraun, becomes Unicorn's lover.
Their sexualities are polar opposites. Quaraun being asexual is content to live without sex, but being demisexual can tolerate being in a sexual relationship. Unicorn has rampant hormones that keep him wanting sex multiple times a day.
Because he is asexual, Quaraun often finds his relationship with Unicorn overburdensome. Quaraun is also very shy, very introverted, and very submissive. He prefers to avoid confrontations and backs down from a fight. This results in his frequently submitting to Unicorn sexually, when he really doesn't want to and his not saying anything to let Unicorn know this is the case. He also feels he can not talk to Unicorn about this and usually turns to GhoulSpawn instead.
Unicorn is a very dominate person. He hates to be challenged, he doesn't like anyone who disobeys him, and he can become violent when he doesn't get his way. He was a high ranking military officer most of his life and expects people to quickly obey and submit to his every whim.
While Quaraun does love Unicorn, he also feels frustrated and overburdened by him. Unicorn has a big personality. He's very bold, very outgoing, very hyper, very extroverted, and often loud and rude, frequently going out of his way to start fights. Quaraun, who just wants peace and quiet, often feels a need to run away from Unicorn and hide.
GhoulSpawn, is a neutral zone. He's not as flighty and submissive as Quaraun, but not as bold and outgoing as Unicorn. Unicorn gets drunk and violent, resulting in Quaraun become very frightened and running to GhoulSpawn for protection.
Though GhoulSpawn is only a teenager, and Quaraun old enough to be his grandfather, you see GhoulSpawn taking a fatherly stance with Quaraun.
Later in the series, when GhoulSpawn is much older, you see him standing up to Unicorn more and more, and a growing bond between him and Quaraun that has Unicorn feeling very threatened.
All of this eventually leads to Quaraun keeping both Unicorn and GhoulSpawn as his lovers, and over time Unicorn's harsh personality softens quite a bit as he comes to realize, Quaraun's love for GhoulSpawn happened as a result of his own chasing Quaraun away.
You eventually see the three of them, always together, never separated, each of the three caring deeply for the other two. THIS is a polyamorous relationship. Three or more people who love one another BY CHOICE. Polyamorous relationships are not forced or arranged.
When a person is forced into a multi partner relationship such as the one you has described, that is called sexual abuse, specifically known as sex slavery and sex trafficking. It is NOT a polyamorous relationship, it is a type of rape.
If you are wondering what I am... I am Transgender/GenderFluid & Demisexual and have identified as such since February 1987, so 2017 is my "official" 30th Anniversary of "being" transgender. I was raised in a Fundamentalist Mormon Polygamist Family (my grandfather had 2 wives, my uncle with the most wives has 5, and my aunt with the most husbands has 8.) There are multiple bi-members of my family, including one of my brothers. Growing up my best friend was a transvestite, and I had 3 other trans friends. As a teen, I was friends with a gay couple, one of whom was a transvestite, both died young, one after fallen ill, the other committing suicide weeks after his partner's death.
These things combined is why my main character, Quaraun, is also a Genderfluid Demisexual Bi-sexual Transvestite living in a polygamist relationship with 4 wives and 2 male partners.