I can say that I've been in a similar situation to your boyfriend's in the case of drunken mishaps.

Me being at college, I was hanging out with my friends on the weekend, and I got drunk. This had always been pretty harmless seeing as it was a small group of friends that I had known since early high school. However, some guy showed up who wasn't suppossed to, and he was an acquaintance who I occasionally chatted up about videogames.

While I had turned over to sleep privately in one of my friend's rooms, he had snuck upstairs to "cheer me up" (I was crying over Brad, my bf living back where I used to before college). He then concluded his little motivational speech by taking advantage of the fact that I could barely see and forgot how I even got on the bed at that point. He came in close, grabbed/turned my face to his and kissed me on the lips. As drunk as I was, it was an immediate reflex to pull away, and I started crying for Brad. The inconsiderate boy then used the excuse that he was just "testing me" and fortunately left.

I immediately stumbled around to find my phone where I called Brad and sobbingly told him what had happened and that I felt so overwhelmed with guilt just by the fact that I wasn't cognitive enough to push the guy away before he was able to make lip contact. Brad was, of course, extremely disappointed with how I put myself in that situation, but forgave me because I was sincere enough to not hide something like that from him. I also told my friend who hosted the party about what happened, and the culprit was immediately kicked out of the apartment that night.

I then promised Brad not to drink without him, and have stuck to it ever since. This event has not kept him from loving me at all or holding it against me ever since we reconciled it.

Anyways, my point is that if what really happened DID infact happen, there probably could have been more effort on his part to reconcile it immediately instead of keeping a dirty little secret, which I'm still not sure I'm understanding this correctly. Alternatively, if it's something that his ex just brought up, theres a good chance she's lying - if he's not lying about not remembering anything, that is. Either way, I believe every woman deserves a faithful and loving man, and he should probably step it up..hm...say.. NOW if he expects to continue with ya!