Do I Stay or Leave?

[Adzz02]

Im currently seeing this guy who i care for loads!!
... But last night he went out with some mates drinking which i have no problem with, and today he told me that he made out with a guy.
I thought nothing of this and said that i wont worry about it as such because you were drunk and if it dint mean anything whats the problem?
However, i later found out that they done more than just kissing (BJ's).

Now i really dont know what to do!?! It feels like he has cheated on me and what makes it worse is that he told me when he is drunk he often does stuff he regrets. And told me he has never gone that far before... This is what makes it worse - He is in a relationship and has gone further than he ever has!?

... What would you do if it was you in my position?
Thankyou x