I don’t think it’s unconventional or taboo at all, it’s one of the most direct/obvious/hard-wired-in-to-humans turn-ons there is. If you like anything about women, femininity, pregnancy is the ultimate hyper-feminine trait. Engorged breasts, nurturing life, it’s kind of what we’re built to be attracted to. I had a vasectomy but my wife and I are both in to the idea of breeding - just never the idea of actually having kids.
I don’t think there is much overlap between “physique from overeating” and pregnancy kink content - they just look different. Part of the pregnancy bump is that it looks out of scale with the rest of the body. That’s part of what marks it out, so it’s the part that speaks to your lizard brain. BBW/chubby content is also totally a thing and also valid, but I think it’s talking to something else, at least in the brains of people who are attracted to women.
I draw a few distinctions. There are “taboo” things which is just anything you can’t bring up in front of your grandma (you know, anything except sex in the missionary position with the lights off with the sole purpose of procreation), which is kind of where any political discussion of sex has sat for a long time. Anything else is just “whatever you do in your bedroom” and exists in a box over there somewhere no one really wants to dig in to.
Anything that would be hazardous to someone’s physical or mental health to practice IRL is taboo for me. Scat play, pretty much anything to do with needles, rape or other non-con where the target is being harmed or held at gunpoint or knifepoint. Sexualizing minors, sexualizing animals. Anything like this. Not that anyone should be outright shamed/shunned but I think indulging anything here is potentially dangerous to at least some degree.
Then there’s stuff I just don’t really understand but is so kind of off the deep end that I don’t necessarily think it’s hurting anyone - I just don’t get it. Giant people, vore, insects or spiders, super inconvenient sex organs (like the size of cars). Most other things, even “alternative” or unusual/taboo kinks like body mods, dominance/humiliation via bodily functions i.e. urination, tentacles, futa/shemales, latex, mind control, all that stuff at least makes sense to me even if I’m not in to it.
The one that’s hard for me to address is incest. I’m not personally in to incest, practicing it IRL seems like it would be insanely harmful to the mental well-being of everyone involved, yet it’s like the biggest kink by content volume on the major porn video sites. I can’t really wrap my head around it, but considering how ubiquitous it is while also recognizing most of this content is literally just captioned in this way and no other effort is made to make it “about” the kink, I think ultimately it’s less about anyone fantasizing about any specific (real or theoretical) person in their life, and instead fantasizing more broadly about someone they would simply have easy access to and is therefore about the literal incest and more about the convenience of… idk, state sponsored GF’s or whatever else. Jury’s out on incest for me.
Interestingly your example of “gross” feet is I think a crossover of something that’s totally unrelated to the body part specifically, and part of a broader kink that involves e.g. smothering in sweat after a workout, cleaning a dirty partner in unsavory ways/ways our brains tell us we know we shouldn’t do because we’ll get sick. Even I think “omarashi” or however you spell the farting/gas kink is kind of wrapped up in this. It’s just taking stuff that our bodies do, we know it’s gross, and sublimating it in to kink territory, even if it’s actually unsafe (however mildly) to indulge in. There can also be a dominance/humiliation aspect here, which again can be detached from both feet and bodily fluids.
I find it interesting you call out facesitting. I think as peoples’ hygiene has improved, we’ve realized you know, the vagina is just a place on the body. The asshole is just a place on the body. If they’re clean, they’re not unhealthy to play with, and you can get a lot of pleasurable sexual mileage out of both. So if you’re clean, why should cunnilingus be taboo? Or even eating ass for that matter? It takes some prep, but so does anal. And if cunnilingus is okay, why shouldn’t facesitting be, since it’s essentially just a specific orientation/arrangement of cunnilingus. Again there can be a “dominant female” attachment to it if you want, and there can be a gross-out/dirty factor too, but those are both independent things that you can choose to involve in the kink or not.