Do Something That Makes You Uncomfortable
Sadly, many nowadays are unable to distinguish between legitimate PTSD triggers or considered moral/ethical lines and simply "being uncomfortable" in terms of appropriate response and so give you carte blanche to skip out on anything challenging. But that's how you never grow as a gamer or a person.
While it's fine to refrain from anything that fits in those former categories, part of the glory of any hobby is pushing you outside your comfort zone - either something as straightforward as doing an Iron Man triathlon when you never have before as an athlete, or in this case something more cerebral and complicated, like crossgender roleplay.
If you're ever going to GM, guess what, you'll end up portraying female characters! And orc characters, and mind flayer lich characters, and all kinds of "different personalities." It's a good skill to learn.
I get that you may be worrying about the group's response - but the GM is the one assigning you the character, and I would bet he probably has an additional agenda of trying to get the people in the group to stretch more, and this is a good way to make it "safe" to do it. It's a one-shot for God's sake, a safe sandbox that's over in one session. Do it, maybe you'll learn something.
You can also see As a man, how can I roleplay a woman better? for tips on how to do it.
When I was a new gamer, I only played same-gender characters, almost exclusively elves, for a while. But then as I grew as a gamer, and did some GMing, I discovered that it was interesting to use gaming not as pure power fantasy but as a means to experience, slightly, other peoples' experiences.
So now I really like stretching with each additional character, always looking to play a different gender, race, personality/alignment, sexuality, class/template/playbook/whatever, and so on, and have found it very rewarding to be able to try to put myself into those other mindsets to gain greater perspective myself.
And I'm part of a group of professional mid-career people who are all comfortable doing it, and any given party we form has an interested set of diverse viewpoints - more diverse than our group of similar-aged mostly-tech white guys would otherwise have. It's made my gaming experience better and I have yet to meet anyone whose experience it hasn't made better.