I (23F) don’t know what to do about my bf (25M) cheating – not cheating?

So hello, I (23F) have been with my boyfriend (25M) for about 3 and a half years now (a bit more) and we moved in together in January. We were friends for 6 months before we entered te relationship. My rule was always that when I’m actively talking to someone who might be more than a friend that i reject everyone (interested in me) so I don’t play with anyone and it’s easier on my mind. And he knew that and he was interested in me first and was really obvious but he didn’t express it directly. So obviously I rejected other men for him. He knew my boundaries. So he has/had this friend group consisting with his friend, his friend’s girlfriend and sister. They were always jocking around and made kind of sexual jokes which I was fine with since I knew about it. I found out this week that he slept with his friends sister when we were in the talking faze and never told me or her brother (the friend). It was after he realised he liked me. We talked about it and he said something along the lines he wanted to tell me multiple times but was so scared of loosing me that he never summoned up the courage to. (Which I joke that he hasn’t had the balls to). He knew I was against any form of cheating whether it was the actual act or some form of micro cheating. I was sooo lost and confused and hurt, I mean still am. Before this I knew he was the one for me and I didn’t know how I got so lucky and was so happy that we had eachother and moved in and everything. I am so close with his entire family and his nephews might I say love me more then him (joking but they really adore me). We talked and talked and I don’t want to loose him but at the same time I don’t want to stay with him if it means I wont have the, not necessarily same but you get the point, love and affection for him because it’s not fair for me and him. I started thinking if I was just overreacting thinking about breaking up, like I know i have the right to be hurt and everything but we weren’t together. Today I found out that a few days prior to us getting official that he sent her a text that she has a nice ass and that it’s perfect for grabbing. They have sexual jokes as a group I get it but now my stomach is just really ready to be emptied. And about one year into the relationship he texted her that he just wanted to ask how was the sex they had that he was just interested in knowing. Which again I’m broken about and asked him why he sent that, to which he said he doesn’t really recall but the only thing that he can think of is that maybe he was insecure about his performance with me so that he asked.

About the intercourse he had he said he was drunk, she initiated (she didn’t know I existed and I know she didn’t mean anything more and it was for one night for her, she’s known for sleeping around) it and he doesn’t really recall half of it and that it was supposed to be a normal get-together with friends and it just happend and he felt like absolute shit after, not only for me but he never had any interest in her and was hurt that it even happened because he valued their friendship. (I mean you get what you sow but alright) Now I don’t know what to think or do. No he really wouldn’t cheat and hasn’t outside of that and he doesn’t have feelings for her, he said and I believed him because he really had no special interactions with her when we were out together. Now I’m not sure because I’m not sure about anything.

This woman is not really in our friend group anymore as well as some other people but we see her sometimes and hang out because again she is his best friends sister.

Some advice to what the frick am I supposed to do.

He isn’t gaslighting me or forcing me to stay, we did talk normally and he wants to marry me and spend his life with me but he understands my feelings and he’s “letting” me choose what I think is the best for me.

Also I know to someone sex is just that a transaction. I am by no means a nun but I knew and know I wouldn’t sleep with just anyone so as so it happens he was my first. So also there’s that.

Help?

Thank you for reading my autobiography and sorry if anything is wrong english is not my first language. :’)

TL;DR; : Boyfriend slept with someone else in our talking faze. Cheating – not cheating? Help