I think it varies. As he said, it can be comfort. For me, it's just a history of bad female partners. I used to let women play men all the time. Years worth of allowing it. But soon I realized that I was sick of 9 out of 10 always pulling the same bullshit that would ruin the RP. This also stopped me from RPing lesbian based RPs because most of the females would do the same things.
Some were just obsessing over their own characters, trying to force me to also play a male midway into the RP so they could randomly have a female character, never contributing to the RP and expecting me to play everyone (then getting mad when I would bring it up to them), and so forth.
So, for me, I was just burned by one too many incompetent female partners. I still try to get back into lesbian RPs from time to time, but...yeah, it's just a total burnout for me.

I also want to make it clear I don't think women cannot play men. I have had some VERY great female partners who could play men spot on. It just tended to go into a cycle of bad roleplays and the occassional good partner just couldn't make up for the overwhelming bad.