First, a lot depends on the age. A little sexual experimentation in one's teens is not unusual, and I am hesitant to start attaching sexual labels on guys that young. I don't know about you folks but I was constantly, and extremely horny at that age, at times almost beyond my ability to control it.
Second, sexual acts do not make a person straight, bi, or gay. Sexuality is about who you are. A gay man can have a night of drunken sex with a woman but that doesn't make him straight. The same goes for a straight guy who performs gay for pay porn.
Third, it is certainly true that a lot of bi men hide behind the straight label, but I think it is extremely arrogant of anyone to tell someone else what their sexuality is. Only they really know. Yes, they may, in fact, be fooling themselves.
Fourth, mere thoughts doesn't really do it either, unless they are regular and pervasive. A gay man can have a sexual thought about a woman here and there and it doesn't make him any less gay. The same goes for a straight man.
Fifth, it depends on what you mean with "fooling around". I've heard some dumb ass statements in my life. For example, two guys comparing dicks makes them gay because only gay guys want see other men's dicks. Or two guys beating off to some porn in the same room makes them gay. Or two guys who share a girl together are, at least, bi, because straight men should be uncomfortable with other straight mens' sexuality.