2nd Chance or End EVERYTHING with him???? Help!!

So I've been living with my bf for 1yr and a half, were in a very serious relationship and spoke about future and getting engaged next summer. We are sooo close and in love, and the chemistry is insane. This is why this problem is even more SHOCKING. We NEVER had any trouble regarding cheating or hanging out with anyone behind eachothers backs. If anything, he was always the needy one. So it was my birthday, we got a table in a club. Invited my friends(25 yr olds) and my sisters friends (21, and 22)It was dark, I was dancing and I turned around and looked at the table, i see my boyfriend sitting next to my sisters friend with his hand on her thigh. Shes wearing a mini skirt, and his hand was just sitting there, like they were a couple?!?! i checked twice to make sure i wasnt seeing things, both of them were staring off in to space with his hand on her UPPPPPPER thigh. I controlled myself and kept it to the next morning. I told him what the fu*k was that, and he REMEMBERED immediately what i was talking about, which made it look even more twisted. I asked him if they were at least talking or laughing or reaching for something, they werent doing anything. So I go see my sister, and I tell her about the sketchy thing that happened at the club. And she tells me "omgggg, i have to tell you something, i didnt tell you cuz i thought she was making a big deal out of nothing and just tryin to get attention"...im like WHAT TELL ME! she tells me her friend complained to her that when my bf saw her on 2 occasions, he'd say hi by kissing her on the neck quickly. Which would be akward but not obvious. She also said she was on her laptop once ( @ my house) and he played with her hair for a while and then walked off. So i flip out i tell him hes perverted and i NEVER expected him to be like that and that it was over. I was disgusted, shocked, and didnt understand?! So the next day my bfs friend comes up to me and tells me something even shittier, that in the BEGINNING of our relationship my bf saw his ex and kissed her, around 2 months into the relationship. The friend and my bf got in to a fight, so he didnt care if he told me. So now i do not know what to do. I saw him two times since it happened, he sent me 1000 texts, his older brother called me saying he made my bf promise him not to mess around again and grow up. hes saying hes never going to look @ someone else again and wants to be w/me forever. And he says he kissed his ex in the very beginning and we werent serious, and that in the club he has no clue why he did that and he didnt hook up with her etc etc. Oh, and i told the girl shes a pretty big bitch for not moving when he was making her uncomfortable, what, did she like it?! I mean if u didnt wanna tell me, at least get up and show him its wrong?! the whole thing is just so disgusting, and the worst part is she looks like a duck!!!!!! So, what do i do? Its been 2 weeks since this happened. Do I give him a 2nd chance so we dont throw everyyyything away (i mean i can walk away whenever i want and i have the upper hand), or do I walk away forever and be single and mingggggggle???