I used to play mostly male characters back when I started roleplaying, but nowadays I'd say I actually have a pretty decent mix of male and female. In the grand scheme of things, I've probably played more males overall, but there have been times when I've realized I'm currently playing more females, too. It really depends, and I've never found myself having trouble playing either one, so this whole "I can't play gender x because I'm gender y" sounds pretty... strange to me, tbh. Not that it really matters, anyone's free to play whatever they want regardless of their reasons, I just don't really get it. Personally though, I don't really consider gender that big a deal either way, and I don't really have a "default" gender or anything.
Heck, oftentimes I pick a gender solely based on which side is less represented in the already existing cast, because it makes it easier for me to decide. So, say, an rp has 8 sign ups for females and only 4 for males - so I'll go ahead and make a male to balance things out a bit, or vice-versa. Or if not, the gender usually comes to me when I think of the character idea itself; oftentimes I start to envision the character's appearance as I come up with their personality and backstory, and that leads me to picking a gender.