Like Mead said, sexuality isn't always black and white. It could be that you're heterosexual, but have a very slight leaning towards bisexuality. As Mead also said, it could be that you're just being more open to realizing that other men are attractive. Doesn't have to be in a sexual way, but your brain is just having trouble deciding where to file that realization.
It's entirely normal to explore and question your sexuality, and no matter where you ultimately feel you fit in, it's fine as long as you are happy!
Since about junior high or possibly before, I had always been generally attracted to both males and females. However it wasn't until college that I was able to solidly identify myself as bisexual. It can take time and experience, don't rush or force yourself into any particular decision.