Why do men on dating sites who aren't transgender mark themselves as women?

Somtimes amiguous, usually obvious. And sometimes even the username, incorporating their real name, is a giveaway.

I would first question how you know a person is not a woman - unless they specifically tell you that they are men, you can't tell by looking at them alone and if talking to them then ask THEM why they have marked themselves as women. I would guess however that in many cases it's just done by accident.