My boyfriend is cheating on me with other men

I know the relationship is over. I'm going to break up with him tonight but god this is so humiliating and my heart hurts.

I caught him before in a way that was not fully cheating but it crossed my boundary enough that we had a conversation about it. I told him that since we are both young (20) and we have been together for 3 years he deserves the chance to explore his sexuality and gender identity if he wanted to. I even gave him the option for an open relationship and that we are still young and if he wanted to explore I wouldn't mind exploring too. In hindsight the relationship was over when I saw the red flags the first time.

He vehemently denied that he was even bisexual (I knew he was lying) and that it was just a BDSM kink. He said that he was terrified to tell me about it and that he wanted to explore that dynamic in our relationship. I gave him 3 weeks of the benefit of the doubt. Now I see the messages talking about being bisexual. I saw the pictures of him cross dressing and calling himself a "sissy" to men on fetlife.

He knows that I'm a queer woman and that I would be accepting of him either way, i've even dated men that cross dressed and eventually transitioned. People used to make fun of me for being the girl that always dated straight men and "turned them gay" and I always tried to brush it off but this just feels like another punch to the gut.

I feel humiliated that I was laying in the same bed as him while he has emotional affairs with other men online. Checking the time stamps on the messages on days I know we were together, he couldn't even wait until I left his house to go send pictures to men online.

I tried to be understanding of his kinks but I realize now that his kink of being a "sissy" is really about the thrill of cheating on me and hiding it. I just wish he was honest with me the first time so I could've had the dignity to leave then. I know I can't change that. I'm breaking up with him tonight I just think I need to get this off of my chest.

I know that i'm stupid and everyone is going to tell me that I should've seen it sooner and I take accountability for that. I'm leaving now but god I still feel so dumb and humiliated. Please be kind I already know how pathetic this whole post sounds for me.