EDV, altho you have the "slight idea in your head that he wasn't able to perform", the intent was clearly there, wouldn't you say? And to reiterate what others have said, the blackout occurs after the drinking--it's not like he was in some sort of zombie state when he went home w/this woman and had no control over his actions.
However, it sounds as if your mind is made up, and of course, only you know the right thing to do for you. I think the advice to take some time and make sure you feel all the emotions before deciding what you do is good--I also understand the urge to forgive and get it behind you.
You know where to find us if you need or want more discussion!