The reason you see that in 1x1 is because so much of it is romance RPing. People tend to do more self-inserting with romance RPs than other kinds from what I've experienced, people also tend to play characters of their own gender exclusively and for some reason hate on those who don't stick to the same arbitrary rule, some people seem to view romantic RPing as a sort of intimate thing (particularly if they like to write out sex scenes instead of fading to black and skipping over them) rather than just people collaborating to make a story, people often have trouble separating the character from the player in their mind (which is part of why the previous issues exist), and people also tend to have a lot of hang ups about their sexuality and how it is perceived by others. Those various reasons combined ought to explain why gender requirements started being a thing, and now the fact that it's a traditionally expected thing in 1x1 interest checks accounts for the people who don't actually fit in one of those categories (or a similar one I didn't think of) or are actually talking about character gender desired rather than player gender.

My own view on the subject is that ultimately the gender of your RP partner doesn't matter because there should be a firm line drawn between the character and the person who plays it, but I get where romance RPers are coming from. Were I to go seek a romance 1x1, I would prefer that the other person be a female simply because that happens to line up with my own sexuality, but if some guy came along who met my writing skill standards and was interested in the thing I wouldn't turn him away because the player gender thing is only a minor preference rather than a requirement in my mind. I play female characters sometimes, recently with greater frequency, and if I were to go looking for a romance 1x1 where my character is a heterosexual female I would still prefer that the person playing the male be a female. This is due to my own minor issues with separating the character from the player, because while I know and understand that they are not the same entity, the thought still pops up in romantic RP things that the other person is male or female, and if they're male that makes me feel like it's kind of awkward and weird because I'm not into dudes. That hasn't ever stopped me from from writing romantic RP things when the other character is controlled by a guy, but the thought of awkwardness is there when the other person is a male and it is not there when the other person is a female. It's dumb and makes no logical sense, but hey, that's the human brain for you.