Am I overreacting for leaving my longtime partner for crossing my sexual boundaries?

Me and my boyfriend (now ex) has been together for five years. He brought up quite early in our relationship that he would like to do butt stuff. I said I don't feel comfortable with that due to previous experiences and that it would probably never happen. Every single time we have been intimate during the years he has asked if he can just put a finger in my butt and I have firmly said no. Nothing goes near my butt. When sex time starts I always say in the beginning that nothing is to be done with my butt.

So yesterday I wasn't in the mood for sex, so I gave him a blow job instead. He insisted that I do it like upside down so my butt would be near his face, if that makes sense. He's obviously incredibly obsessed and turned on by butts. So I was like okay, but if I do that you have to promise me to only look and not touch. He agreed and I trust him because I mean, we have been together for quite some time. But then I feel a finger going into my butthole. I just freeze and he continues until he's coming. I couldn't move or say anything, I was so in shock. After he came I broke down crying hysterically and screamed "how could you do that?". He said he thought he had consent because I moved in some way that he apparently took as consent?!

I am beyond furious. He apologised a lot, but said he really thought it was okay. I asked him how the f* he thought something I have said for five years I don't want to do suddenly would be okay, and he didn't even ask first. He never once checked in with me. He said since I felt like I was relaxed I was consenting. I told him that the body reacting in certain ways doesn't mean consent. Obviously?! He said I could have just said no and he would stop. I was furious for him trying to shift blame. It's totally on him to make sure he has consent before he does anything to another person I said. And I froze, so I physically couldn't.

He said it was a mistake and begged me to forgive him. I don't see it as a mistake at all. He can't be that stupid. I told him I don't feel safe with him any more since he actually sexually assaulted me. He got super upset because he "isn't a rapist" and how could I accuse him of anything like that. He tried to make it all about him and his feelings. I took my things and left. He says I'm overreacting for a mistake, but again, can you really be that stupid?