When I was in college for a while I rented a room in a house off campus. Another guy rented a room there too. I didn't know him, didn't even talk to him and rarely saw him. When I did he'd have a girl with him.
One night I was in bed and he just opened the door and walked into my room, crawled in bed with me and we started to make out. We then sucked each other off. Never said a word, only happened that one time.
Also during college I worked at a YMCA. Became friends with a guy that worked there, we'd hang out. He eventually asked me if I was gay, and he was cool with it. He told me, as a kid, he'd jo'd with his cousin. He wondered what it would be like to suck cock and get sucked by a guy. So, we did. Only once, and he said it was good, but that he wasn't gay, just wanted to try it. He did get married and had a family - we still keep in touch.
I know guys say that if a man does something with another man he isn't really straight. I say there are cases when they are, just open to trying something out - but they primarily are straight men. Just like a lot of gay men have had sex with women - they aren't bi, or staight - they're gay with some experience with the opposite sex.