|Author's note: You delve into some controversial topics and standpoints, but I believe your story can open people's minds on your viewpoints, primarily since you base elements in it on your personal experiences. You also state that this story is an OC x canon fanfiction, which provides other readers and me the opportunity to critique your OC, whom I will assume is Joan. Speaking of her, she's the only character I'll focus on because I know nothing about Seito Kaba or the series to which he belongs.
Chapter 1: Your accounts of Joan's experiences reflect how society expects women to act as prostitutes and escorts very well. Providing insight into Joan's thoughts and surroundings make your accounts of her perspective seem more personal, which aid in your goal of giving personal insights on what it's like to be a prostitute. However, from what I've gathered from the context, Joan is expected to be quiet and look pretty; for example, she has no chance to defend or explain herself when Marc announces out of nowhere that she has a brother fetish. This information from the context gives off the impression that prostitution isn't an empowering choice of employment for women, which would turn specific members of your audience against your favorable position on prostitution. With this said, it would help to add contextual examples of how prostitution can be empowering to women.
Chapter 2: This is the chapter where I got to meet Seito as a character. Although you wrote him well, I do not like his personality. While a well-written character doesn't need a likable personality, I believe that Seito's nature plays a big part in influencing whether the reader will agree with you on your stance on prostitution, especially in his first interaction with Joan. When reading how he looked at Joan's body and requested she come to bed with him, thinking about how he prefers prostitutes to other women because they "didn't simply go through the motions" out of fear of ruining their makeup in the process, I gathered that he values women primarily for sex. I would not want a man to view me like this, and neither would a lot of women. I would have liked to know what was going through Joan's head as she stood there quietly, passively taking Seito's disrespect. What were her first impressions of Seito? Did she like the way he treated her? Why would Joan agree to come with Seito?
Chapter 3: To be honest, I couldn't finish the whole chapter; I read the line where he got an erection and got too scared from there. For this chapter, I can't say much except that Joan appears to be more comfortable with her boyfriend, especially in how you describe her interactions with him through the context. This chapter seems to me where the theme of polyamory comes in. Since I didn't read the whole chapter, I can't say anything substantial about how well the plot events persuade your readers toward your perspective.
Rest of the story: I honestly couldn't read to the end. I thought things would get better in Chapter 4, but I lost hope after Marc told Joan he was going to rip off her new dress right after she came back from Seito's place. I feel like all the men around Joan treat her like a sex object. As far as I know, or at least as I'm choosing to assume because I don't want to see Joan as a Mary Sue sex whore, Joan is supposed to feel safe with Marc, yet she lets him talk to her like he does. She also says nothing when Seito, a man she just met, talks to her like she's a sex object right off the bat. You say that you based Joan's experiences as a prostitute on your own, yet I'm having difficulty seeing that in your writing since I barely find any input on Joan's inner thoughts and feelings at all.
While the story as a whole is well-written, I feel like it didn't serve its intended purpose of portraying prostitution and polyamory in a positive light, mainly due to how the male characters treat Joan. You could've kept this element and still managed to relay an empowering message about prostitution and polyamory if you had given more insight on Joan's thoughts and feelings, especially regarding how the men around her treat her.