I think some people feel a little more comfortable around the opposite sex or that they feel they will be more sympathic towards them.  Of course there are people that aren't here to actually learn a language and use this site more as a flirting/dating tool, but if you are really here to learn, you shouldn't really take one gender over the other.  Your level, likes/dislikes, personality, and age are far more important criteria than your gender when looking for a language partner (at least a long term one.)

Well, since this topic comes up every week, allow me to twist it and return it with another question:

Why most people WOULDN'T prefer to have a language partner of the opposite sex? What's the problem with that?

I find women more patient when it comes to correcting your spoken language skills, but I don't mind talking with other guys. Indeed, I don't care about people's gender, religion or age as long as they can speak the languages I'm learning fluently and flawlessly.

Suppose it's the old rule: opposites attract. We bring different things to the table. I really want a female language partner, though--someone who understands being female from experience. Too much testostorone at the moment. xDD

I don't really know, but I have contacted with some girls to practice my English, and most of them didn't answer... :/ Have to say that when a person just wants to learn or practice the language it can be seen!