My boyfriend cheated on me with my best-friend.. how do I get through this?

[Mando Pandango]

we've been together for 1 & 1/2 years now. He lost his virginity to me in the beginning of our relationship, the cheating happened about 3weeks ago. The three of us were all drinking & it was close to bedtime, when I told my friend she could sleep with me since I promised not to leave her alone. My boyfriend was going to sleep beside me no matter what, but I trusted her and him enough for us to all sleep together that night.As I went to the bathroom, I got a bad gut feeling. Turned out he began to rub her private area fully clothed,then my friend adjusted his hands inside her pants. When she saw the bathroom light go out she quickly removed his hand and called him names, so it appeared that she rejected him and it was all his fault.I heard her call him names and overreacted, punched him a few times and left. I came back 5secs later, remembering she was still in his house when I clearly didn't want her to stay there. He claims at this point he was in shock at what he and my friend had done and was sitting on the bed in awe that he had lost me. When I went back inside I punched him more, and ordered my friend to leave with me.As we left, my boyfriend smacked her bum as we were out the door and she turned and smiled at him ( which i just found out from him a few days ago). Everything I just wrote is my boyfriends side of the story. My friend, on the other hand, claims that he only tried to kiss/hug her while I was in the bathroom and after he was caught ( when i left ). . but she says she rejected him and that everything else he claims to be true, is a lie. I cut her off completely, not believing anything she said. I decided to give my boyfriend another chance, considering he told me the truth, and this is the only time this happened, but I am still having troubles dealing with this. He is doing everything he can to prove himself, and is willing to do anything to help our relationship and my insecurities. What should I ask of him? I can never forgive my friend for this ever & I want to marry this guy, but my insecurities are getting to me. What is the most logical thing for me to do? What can he or I do to save this relationship?

[spoiler]This was a spambot post on another forum I go to and I found it hilarious[/spoiler]

[PLEASEHELP1991]

I guess Applesauce wasn't a good boyfriend for you.

[snoorkel]

Turned out he began to rub her private area fully clothed,then my friend adjusted his hands inside her pants.

I came

[Wrench]

I came

prematurely

[Socks]

There were terrible decisions made at all levels of this story. The terms they use to describe themselves and their situation cannot be compared seriously with mine, because their concept of what they mean as shown by their actions is extremely shocking and limited. Thus the end is not surprising, but it is still disturbing.

[Samus Aran]

i don't think spambot messages particularly worry themselves over interesting word choice