It's hard for me to recall such origins that could go back 15 years or so, because so much of my journey of understanding myself sexually has been solo, and it blends together. (Off-topic: I dove deep into femme stuff with my first live-in gf and that partly derailed the relationship. Subsequently, I kind of compartmentalized the femme part of myself from partners even when I probably didn't need to, maybe self-policing subconsciously.)
In asking myself this question, I see how my journey was shaped by cisnormativity and not having many close queer friends who I could talk to about sex and kinky things. This is as good a jumping off point as any:
I was into "whaletails" - that is when a (women's) thong rides up over the pants (trousers) and is visible to passersby in public. I probably came across this term randomly through porn. But my interest in it persisted. I browsed forums about it. A little while later, I wore a thong. And I posted staged whaletail pics of myself, still while on a whaletail-specific forum iirc. Then, I think a couple years went by and I took some time off whaletail stuff (post breakup mentioned above), and then I return to it. Reddit's now bigger. I probably found sissy stuff on Reddit through searching for whaletail stuff online. The feminization resonated. About 10 years after stumbling on sissy stuff, I'd pretty confidently identify as a non-transitioning trans person; ie, "I'm not cis I just play it most of the time in real life." :P
I feel I haven't really been about to think outside the (cishet) box. A lot of my discovery of what I'm into has been through finding "oh, there's an online community for there. There's a term for that. Yeah, I think I'm into that, I just never thought of it."
Another thing: I had no more than a 2-sentence understanding of D/s stuff before learning about sissy stuff; I learned about bdsm through sissy stuff. I feel like that's weird.
I guess I wish I could give some genderqueer person a hug and say, "it's okay, there's more than one way to have sex," and I think I will do that π₯°. Good question!