Okay so I read the whole thing. Let me tell you my thoughts as I went through:
because they didn’t like to see us engaging in acts that they could perceive as either romantic or sexual. They would get frustrated and feel the need to leave or pick fights with our partner if it felt like I was talking too directly to them, if we kissed, if they were too sweet to me in one way or another.
So my thought on this was that it's an okay boundary to have. The way one polices this boundary for themselves is one limiting the amount of time one spends with their partner and metamour together. So I wouldn't expect someone with this boundary to spend a lot of time in their partner's nesting home.
My partner started policing the way they interacted with me because they felt like they were making them upset, and in turn, I started to feel neglected.
I'd certainly bear my partner's needs in mind, but I wouldn't agree to them spending lots of time in my home with my partner where we had to "mask".
or they would engage in heavy kissing sessions with me awkwardly there,
If your metamour didn't want to see this, it's surprising they didn't mirror the behaviour they want to see. This is where the imbalance starts to show.
Over time, a few serious fights broke out because it felt like I never had any time alone with our partner, and that my metamour was here most of the week, spending as much time with them as they could. Whenever I did get private time with our partner, either they had a lot of work to catch up on, or my metamour would miraculously be going through something really difficult that demanded a lot of attention from our partner over the phone or via text, so it basically felt like they weren’t there with me at all. It was extremely rare that we actually got to spend time together that felt genuine.
I wonder why your partner was not also craving quality time with you.
I have expressed numerous times that would like the house I live in to remain mostly sex free,
This is not a need compatible with most poly people, I'd say. This is where I start to think living together just isn't a healthy option for either of you.
They have other places they can sleep together,
Your partner may want their home to be somewhere they can have sex with all their partners. This is what I mean about compatibility.
My comfortability with sex shifts a lot, which is why I initially wanted to try poly relationships with my previous partners. I felt like it provided a way for me to receive the romantic connections I crave while also allowing my partners to get the sexual gratification they deserve from other people when I was unable to feel comfortable doing so with them.
I strongly believe that sexual incompatibility is a bad reason to open a relationship. It's not a fix to prop up an established relationship.
After expressing this, it feels like my metamour didn’t really understand the depth to which this would hurt me.
Or your metamour feels your partner can make their own decisions about whether to have sex in their bed and your needs aren't more important when it comes to shared space. I agree that neither of your needs overrides the other - it's simply not a sustainable situation for either of you.
but after talking with a therapist, they suggested I not try to police my partner’s actions anymore and just let them do what they want, because it’s apparent that it’ll happen regardless.
It sounds like your therapist said this because moving out isn't an option and you really can't have total say over shared space unless you have some upper hand thag forces your partner to relent to you when it's an impasse.
My therapist suggested I stop trying to see the apartment as a place where I can feel “safe” and start to think about it as a public place we all enjoy together. This way of thinking has helped me not feel so stressed about the sexual stuff, but it does make me feel like I have nowhere to retreat to that feels safe and comfortable.
As you said, you need to create that space by moving out.
I feel like a terrible partner for trying to enforce or restrict the actions our partner engages in.
You're not. You're entitled to feel safe in your own home, but you have to be on the same page as your housemates (partners or not) for that to occur. There has to be quite high levels of compatibility in several areas. You and your partner don't have that, at least as a poly couple.
Your needs are perfectly reasonable. You don't need therapy to feel differently and many people who have no history of sexual trauma feel the same as you while managing healthy poly relationships that respect those boundaries.
The answer is that you live separately and you've identified there are obstacles to doing that at this minute. But see that as the light as at the end of the tunnel. You know you're working towards that so keep going for that goal.