<human>: Can you provide a brief overview of the text?

So my buddy is telling me that he slept with my girlfriend. I'm a sex-positive, open-minded guy and that sort of thing doesn't really bug me so much. I mean, it's understandable. He's a good-looking guy, she's a hot girl, they were drunk, and he's saying they fucked in the back of her car.

I found this out because she had been trickle-truthing about the issue for the last couple weeks. What started out as "him trying to kiss me" and it happening "a long time ago", turned into "him kissing me" and it having happened a few weeks ago. My red flags were up so I asked him about it and he said they slept together. 

When I talked to her about it, I was open, honest, calm, and rational. I explained that I knew what had happened between the two of them and that I just wanted her to not feel like she had to lie about it. It was the trickle-truth lying that hurt. During our conversation, she admitted that they had actually made out. Even more trickle truth. But she adamantly denies having sex with him.

I've made it clear that I want to be there for her, that it's ok to be honest with me about it, that I'm not going to judge her for it. I just want to have the truth. I tried to create a conversational environment in which she could be honest and vulnerable with me. Ultimately, I told her that because she lied about it from the beginning and the truth trickled in, the story getting worse each time, I couldn't believe her and I don't. I just want her to be honest with me, but even though I'm a safe person to be honest about with it all, she denies having slept with him. 

So now I'm going nutso. I don't know whether my buddy was just trying to be douchy and manly and made up sleeping with her (he's *that* type of guy) or if he's telling the truth. 

How can I know?
<bot>: my buddy says he slept with my girlfriend. My girlfriend says that it never happened, that they only made out. I don't know who to believe.