A slang term for a man who is more sexually attracted to other men than to women. It is not usually derogatory but it can be sometimes. It has the connotation of a gentleman who is educated and refined and who does not "broadcast" their orientation.
"I don't think Chris wants to have sex with you Lauren - he's sly."
"I was totally plastered and I wanted to have sex with this really really hot guy, but it turned out he's sly, so he just walked me back to my dorm instead."
"I can never make friends with guys unless they're sly. Otherwise they are always staring at my breasts."