Never had the opportunity, I shy away from committed "relationships" of any kind with a girl, I try to keep things purely "sensual" because I don't want a "relationship" right now! After college, not now!
But to be hypothetical, if I was, I may or may not cheat, depending on the girl and opportunities that came my way. Probably not, since I'd only be in a committed relationship with a girl I really like (i.e. not stereotypical teenage pleasure object!) and thus would not consider cheating on her, hopefully, but who's to say... Every time I think I'm sure of what I'm capable of, I surprise myself, for better or worse...