If your boyfriend seems to be fixating on men, he could very well be gay. Especially in a world that's still not very accepting of sexual orientations outside the straight community, it can be terrifying for people to own their homosexuality. That's why some people hide it from those close to them.
You may find him secretively admiring the physique of another guy - even just a lingering gaze or a man-to-man touch that feels weird to you. Or, he's talking about his friends who are gay and the things they do together that turn them on (for example, "I'm really into oral sex," or "I love getting a deep penetrating").
He seems to be more interested in learning more about other men. This could be a sign that he's more open to being gay and wants to be more open about his orientation with you. Alternatively, he could be fearful of the societal prejudices that are associated with being gay.
He's a huge fan of celebrities who are known to be gay. For instance, he might mention Cher or Paul O'Grady with awe and reverence, and not just because they are famous singers. He's also a big fan of actors, directors and other entertainers who have been known to lead gay lifestyles. He's not just a fan because they're popular; his interest could be because they lead such a unique lifestyle, with an interesting mindset, looks and habits.