My partner cheated on me, then told me it was okay because we're poly now, and decided that I'm involved in the new relationship

I somehow got involved in a polyamorous relationship and don't know how. I told them I didn't want it but now they say we're all in a relationship. I don't want to hurt my partner by telling him he can't be with her. I was comfortable with swinging, him and I together. They ended up crossing a line and cheating on me by having sex together and not telling me for months (then justifying it because "we're all dating each other"). I feel scummy but found out by snooping through my partner's phone. I'm trying to forgive but I can't trust them and they keep crossing lines. I don't know what to do.

You did not “get involved in a polyamorous relationship,” you got cheated on. Full stop.

Imagine if you got home one day and your partner had packed up all your stuff and said “we’re moving, you live in Italy now!”

Imagine if your coworker moved all your stuff to the front desk and told you “you’re the receptionist now!”

Imagine if your neighbor stole all your stuff and helpfully informed you “you’re a minimalist now!”

Imagine if your sibling threw out all your food because “you’re a vegan now!”

Imagine if your friend brought over some rando you’d never met and said “meet Ferman, he is your boyfriend now!”

Those situations above are absurd. Someone cannot just decide you into a situation you didn’t consent to. Your partner is cheating on you, gaslighting you, and being completely manipulative. Just because they have some weird justifications doesn’t mean you need to accept that. Stop “trying to forgive” and leave this relationship immediately! You are not being treated acceptably, your partner is not respecting your personal agency, and this relationship has been irreparably violated. I am so sorry.