Perfect! So if she was assaulted, her first move needs to be going to the police.

Are we now excusing the fact that she intentionally cheated on OP? I'm just saying, right now we have the word of OP's girlfriend, someone who at least cheated on OP once. Also, what did OP mean by "(or pretended)" when talking about whether the girl was asleep?

Do you buy that her girl friend woke up, saw her getting sexually assaulted, and then initiated sex? "Oh my god yeah, you just got sexually assaulted right in front of me! Let me try to have sex with you."

Plus it's entirely possible that she assumed (correctly) that OP would better forgive her for cheating with a female.

I'm not saying she is lying.... I'm just saying that there are definitely oddities to her story that don't make much sense, and "he said/she said," especially when the girl cheated for sure that same night, should not be enough for us to just assume OP's SO is telling the truth... you see that as "not fair" to OP's girlfriend, but I don't see how acting like the guy definitely assaulted the girl, especially considering the details, is fair at all.