For myself, it's a simple, selfish, and stupid reason. I like romantic plots when I do 1x1s, so long as there's still plot. I tend to get heavily invested in my characters emotionally, and it tends to bleed out a little toward the RPer I'm working with, even though it's not intentional.

But if I'm RPing with another male, when they're RPing a female character...

Yeah, that just would get weird.

Plus, I'm looking for someone to socialize with as much as I am someone to RP with, and I already have enough male friends. >_____>

As I said, Simple, Selfish, Stupid.