Oh, it's more to do with characterization than comfort levels. It's just more difficult to create an NPC who, for instance, appeals to the character as a woman without putting off the player as a man. It's more difficult with male players, for me at least. I'm curious if the opposite is true for male DMs, but darned if I knew where to find them.
I find that the problem with NPCs is simply that they try too hard. Men are either charactichures(sp) of real men, or they're obsessively one thing. If they like fighting then the LOVE fighting, if they like food then they're fat and the OBSESSED with food. And so on. Or worse, they're men who approach romance like women, with subtle clues, dropping hints, and so on. In short, simply due to time constraints, male NPCs(IME) have generally been about as 2-dimensional as you can get.
Not to mention that even though the PC is a woman, the player is still a man, and men and women have distinctly different ways of picking up that someone is trying to romance them. Their brain simply isn't trained to receive information in the same way.
As a man, I can honestly say that as much as I like the shapes the female body provides, any other man can be funny, romantic, love food, enjoy heavy metal, and therefore, make me somewhat attracted to them. But I don't think most DMs, even the good ones, are going to invest that much time in it.
Not to mention, all relationships are willingly entered into(unless there's money!) which is why player-NPC romances are generally bad ideas. Either a lot of good work goes to waste when they're spurned, or it just gets weird essentially romancing the DM.