What does a straight guy gain from befriending a gay guy?

[Sky]

Exactly the same as if he befriends a straight guy: he gains a friend. Duh.

You can't judge that guy's sexual orientation only from his appearance and style of dress. There used to be a guy who worked at my company who I'd swear was flaming gay, but he had a wife and kids. Ditto for my former barber, although he had a wife but no kids. Just be friends with him, learn about him the same as you would with anyone else, and gain some understanding about who he is as a person. That's what being a friend is all about. If it turns out he is gay or bisexual and tries to move the friendship in a way you aren't comfortable with, just tell him that you're not interested in that and you two can only ever be friends, nothing more. But until/unless that happens or he actually comes out as gay or bi, don't make assumptions about him. If you're compatible as friends then just accept him for who he is.