Short answer is: I'm not really sure, anymore. Some combination of wanting to be adorable, wanting attention, some kind of disconnect making me it easier to disassociate my character's actions from my own personality, and habit.
Long answer is,
When I first did it for much time at all was on a Neverwinter Nights 2 roleplaying server. I think some odd, non-destructively sexist part of my brain decided—if people expect women to be less capable or accomplished—anything a female character does is slightly more badass. Like rooting for the underdog. I can't recall if there was anything more to the decision than that. I made damn sure that if I noticed any sort of preferential treatment based on my character's sex, it was reciprocated or refused… but then before very long, someone expressed romantic interest in my character. As a sexually insecure, at-the-time presumed straight teenager, I don't know what motivated me to uh, entertain his advances.
Like, vag-wise.
In any case, I enjoyed it. At the time, I think being a different sex, being differentiated from my actual identity, made the whole exercise kind of liberating. And then I rerolled on an ERP-focused server with a much less conservative character. It's been close to a decade, but I still kind of miss it.
Of course, at the same time I was bedridden with chronic fatigue syndrome, and diagnosed with hypersexuality. I didn't have much opportunity to interact with girls, and tits were a good start to attracting attention in the ERP arena. It wasn't until playing an MMO later that I outright claimed to be a girl in real life, because people were more social, and I was the only one still hiding behind a character. One person gave me in-game currency in hopes of getting text-based blowjobs (and romance). Which he did, before long, even with me discouragingly saying I really wasn't looking for a relationship. I think I enjoyed the kind of guilty rush of that transaction than any use I got out of the gold. Even so, probably a low point for me. It wasn't until I learned more about him, and that I was probably disrupting the life of some random guy with an actual life outside of his house, that it really hit me that maybe what I was doing had any sort of consequences and was worth examination. But that was a while ago, and it still hasn't really gotten that examination.
I do a lot less of it these days and take breaks now and then, but sooner or later I always kind of want to come back to it, and for some reason I have a hard time getting invested in any kind of roleplay if my character is male.
So I think it has something to do with wanting attention, with idealizing women as this sort of mythical beast carrying a non-existant get-out-of-jail free card for awkward social situations, and with habit. These days, I dunno what it all means. I kind of doubt a shocking twist where it turns out I was trans all along. Not even sure if I'd call myself bi. I think I want to be smaller, cuter, and cuddlier than I ever actually will be. I think I like dicks, but not really the rest of the male form. If I stumbled across a gloryhole, I think my most compelling reason for not playing along would be that I'm probably kneeling in filth.