1) Guys are just fucking horny.
That says it all right there.
A over a decade ago, when I was in my early twenties, I was chatted up by a gay bartender at my local pub. After a few drinks, he had my curiosity piqued, and I went home with him. It was quite the experience, one that I'm glad I had, and would probably repeat if I ever had the chance.
I was never confused about being bisexual or gay; I love women far too much. Instead, it was more of a purely hedonistic experience, which brings us nicely to the quote above; 'Guys are just fucking horny.'