I would play out a scene in your mind as if you were watching a film.
He was pissed. Sweaty and talking shit.
She was the same and the pair of them were alone and started fumbling around. The smell of bad breath of what they had bothered eaten and drunk.
Horrible drunk snogging with lots of saliva and grabbing at each other. Knickers off, trousers and pants down. Like two fish floundering out of water which probably lasted only a couple of minutes and then it was all over.
That's my take on what probably happened and quite frankly it sounds revolting.
Don't feel your head that it was romantic or done with emotion or either caring about each other.
Alcohol lowers inhibitions and it's no wonder he feels ashamed.
The immediate worry for him should be that he's caught something or got her pregnant.
He needs to get himself tested for any STD's.
Personally I could not forgive especially if he is denying it. At least if he came clean and was apologetic and owned up to his failure of morals and respect for you that would put him in an ever so slightly better light but no he is insulting you further by denying it.