There is nothing inherently feminine about peeing sitting down, and it won't make you gay.
It doesn't sound very manly to me, on the other hand, to allow your mate to manage your personal body functions for you. It would make a lot more sense to me if she simply asked you to tidy up the bathroom after you used it, rather than ordering you to pee in a certain way. But she's a bit of a micromanager, isn't she?
Sitting down won't solve your problem, but standing up for yourself might.