James, that is messed up! I really have no idea what to make of this mess. What does guy #2 have to say about this? Is he the "it's not mine!" guy?
BTW, pregnancy is the result of one sexual experience. It doesn't matter how many or few times someone has done it. It's a matter of timing.
I think that the girl should be honest with both of the potential fathers. I doubt that it will happen, though.
I can see why men don't trust women particularly this woman. No one seems to have behaved too well in your story. Even the BF leaves something to be desired. "So Mr. Ivy League, why are you getting marrried?" "My Girlfriend's pregnant and it is the right thing to do."

I feel bad for him.
This situation is a recipe for disaster or an appearence on Jerry Springer.