When I play a team mode in multiplayer, it's like 80% of all games I join are female vs. female character teams. I don't mind playing as a woman from time to time, no problem. But I got the feel this is some kind of "proof your manliness" thing like some players find it hot seeing two teams of chicks hunting each other down. And the overwhelming amount of these intentional Kill-Porn-Games are really annoying, and keeping me away from team modes. Shame on you.
I feel absolutely no shame playing as the female characters in this game, nor do I see any reason why I should, nor do I see why the OP would make some kind of sexist/porn-related leap to assume there's something sexual behind it? Personally I just think the female characters are better. Their customizations are better, their champion packs are better, they're smaller so it's a little easier to hide, etc. Plus...my other main game is Gears of War so I play games as super burly dudes all the time. In fact that's MOST games. It's nice to play as a female character just to mix it up.