There is this interesting idea that says:
Man: "I'm not gay, I just have sex with men."
Being gay means that you are willing to enter a relationship with someone of the same gender, not just engage in sexual activity. As humans, going by Darwin's ideas, are animalistic, sexual drive is a natural response. . .which makes sense that most people will, at some point in their life, be attracted, at least physically responsive, to someone of the same sex.
As for being straight, gay, or bi, that comes down to preference in dealing with relationships.
Personally I identify as pansexual, for the sake of argument I'll say I'm bisexual.