So, I can completely understand how some people only want to write with a partner of a certain gender. Like, if you are a man, and want to do some collab with a woman on some sexy writing, it's part of the prospect to know that you're writing with each other is gratifying the other person. I can totally see how someone who is straight would be weird about getting another person of the same sex all hot and horny over their writing. I understand. But I'm not adverse to writing with female partners, myself. But then again, I'm also not exactly straight. lol I've been sexually active with both men and women, so it's less of an issue with me. Fundamentally, it should matter more if you are actually a good writer. And whether your interests and styles mesh well. That should be a bigger focus on how the roleplaying gets along.
Here's the real question though... how would you even know? Like, some people are very good writers. To the point where they might be very good at narrating the opposite sex. And some people might bumble writing even their own sex. So, what's to distinguish if the person you are writing with even IS the gender you think they are? This is the internet after all.