A triangle may be forming. Help me see it?

Alright, I find myself here again, asking for sincere help because I found everyone to be helpful and friendly. Well, on with my question...

I started dating this girl who was originally a friend of mine. Since high school ended I found that we were slowly getting closer. Eventually we became a couple. She is a mutual friend of me and my best friend. What I didn't know is that she originally liked him in high school but stopped because he was the boyfriend of her friend. The week before she and I became a couple, my best friend brought her to the beach. Just the two of them. And the idiot asked her to kiss him. Multiple times. (This right after he started his, "break" with his girlfriend.) She told me this a month after we were a couple and claimed to have denied him the pleasure of kissing. Upon hearing this I lit up on the inside. My best friend. Was this stupid. I know it was before we were even together. But I felt pretty betrayed. When I approached him about it he kind of played it off. "I don't remember why I asked." And tried to go off telling me that he was trying to help me. I just brushed it off, thinking it doesn't matter now that she is with me. But it bothers me now. Can I trust either of them? Do I need a new best friend? How do I ensure myself that I can trust her to stay faithful and trust my friend to not be a complete moron. If I can stop it, how would I do so without causing blunt force trauma to his head? I know his past and he doesn't make any good decision when it comes to girls. Having attempted to cheat on his girlfriend and nearly getting thrown in jail for statutory. Is this a guy I can trust with my girlfriend? He is a nice guy and I don't see him as the type to betray me because we spent two years as best friends but I notice how poorly he makes decisions with girls. A true amazement how his girlfriend hasn't broke up yet.