I'm not sure if he has been faithful. What do I do?

Conflicted :-(

I have been dating my boyfriend for 6 months now and we get in little fights occasionally, but on the whole everything seemed good. I was happy, and I guess I thought he was too because we spend almost all of our free time together, I see him every day and he's never complained about anything. He gave me no reason to believe that he doesn't want to be with me anymore, and if he didn't he is the type of person who would have broken up with me by now.

Anyway, a month ago I let him go to a party with his two guy friends while I hung out with some of my friends. Later that night I got a phone call from one of his friends who went with him and he asked me if I was mad at my b/f. I asked why because I didn't have a reason to be, and then he started telling me that at the party he danced with a girl and she kissed him, but he pushed her away so I shouldn't be upset about it. Then my b/f called me when he got home and asked me to come over because he missed me, so I did because I wanted to ask him about what his friend told me. He admitted to it, but I wasn't mad because it was just dancing.

The next day I got a phone call from one of my friends who tells me that another one of our friends told her that the girl my b/f danced with, who happens to be close friends with her, had sex with him that night. I immediately confronted my b/f about this and he told me that he doesn't think that he did it, but he can't give me a definite answer because he was so drunk he doesn't remember everything that happened that night. I talked to both of his friends, one of which doesn't drink, and they both said that they are sure he didn't cheat on me because at least one of them was with him the whole night and it was another guy who walked the girl home.

I was really upset at the fact that my b/f could not 100% deny cheating on me, and I considered breaking up with him but he begged me not to. He told me he loved me and he would do anything for me to stay with him. After that incident he began acting like the perfect boyfriend, and I put that night behind me. I even got the girls number, but I never called it.

Now just last night one of my best friends told me that one of my b/fs friends told our other friend that my b/f has been cheating on me from the beginning of our relationship. He said that he had sex with the girl at that party and that he has cheated on me at his place, where a lot of us frequently go to hang out at. He made it sound like I should be worried about getting checked for STDs and I don't know what to think. I asked my b/f about it and he denies everything, he even said he wanted to beat up his friend for saying that stuff. I don't really trust his friend, but now I'm starting to doubt myself.

I don't want to break up with my b/f because I love him and if he's not cheating then it would be for no reason, but I don't want to be the idiot that is being played and cheated on while everyone knows about it and just talks about it behind my back. I don't think that my b/f is the kind of guy who would do that to me, especially after what happened before when I thought he cheated and how he responded to the threat of losing me.

I don't want to start any drama or involve other people and I don't want my b/f to be upset because he thinks I don't trust him, but I need to know if it is true because then not only do I need to get checked for everything, but also that means our entire relationship was based on a lie and that he has been lying to me the whole time. And he's a horrible liar. What should I do? Should I just forget about it and act like nothing happened or should I start asking questions and try to discover the truth?