Being a man means having a cock. And that cock is a pretty big part of your life, not to mention your sexuality. Why is it so strange that men, straight or gay, are fascinated by cock related stuff? On top we live in a culture that connects size to masculinity and sexual potency. And so many men are insecure about their endowment, what is normal etc. with small opportunity to compare with normal men, not porn.
I would consider a straight man that had no interest in cock related stuff a bit odd. And it doesn't mean we are all gay. We don't necessarily look for sexual interaction with men. The magic of having a cock is universal. I want to explore it and use it when I'm with women.