I'm glad people are clarifying what they think it means because yesterday when I made all those polls I had my own definition in mind (that fair game means you think it is perfectly fine to chase after somebody when they're involved).
The reason I had that definition in mind is because a guy who [used to be] my friend thought girls who had boyfriends were "fair game" (his exact words). And he ALWAYS seemed to be crushing on girls who were taken! With one of the girls he was hanging out with her one on one and with groups. Actually, now that I think about it, there were two girls who had boyfriends that I can think of where he did that. There was a pic of him and one of the girls on facebook, drunk, together in a bar WITHOUT her bf being there. Anyway then one day one of the girls got engaged and he wrote me this long depressed (pathetic, I might add) e-mail. Also, not surprisingly, this guy didn't respect my relationship when I was in one.
As a sidenote I think the girls who hung out with him alone in the examples above were pretty crappy girlfriends.
Then there also have been 2-3 guys who, when I've had boyfriends, have without a doubt tried to steal me away. One guy (this was in highschool) was like calling me and writing me poems and crap -- and he had met my boyfriend and hung out with us and my other friends! And another guy (this was in college a few years ago) was always calling me and blatantly asking me out (more than once) even though he knew I had a boyfriend. And I'm not even going to go into what the third guy did.
So yeah in recent years when it has come choosing my friends, I've been sticking with girls. I don't hang out with any guys anymore (with the obvious exception of Kirk).