Mpreg? A niche fetish? Have you ever looked at fanfiction? It's a fantasy for gender-nonconforming people of all genders, of queers and bisexuals, of exclusive homosexuals... there are entire distinct GENRES of mpreg (I'm looking at you, ABO), and one of the specific genres is "men in space future reproducing without the middle(wo)man.
There's significant fetish overlap with involuntary transformation, bimbofication, and sissification but without the uncomfortable nature of losing your personal identity, because pregnancy hijacks you without overtly forcing you to become Something Else Entirely. It's a status condition, not a transformation, essentially, and also hugely tied in with BDSM power dynamics, which is gonna appeal to people into BDSM too, even if they're not especially fans of preg content. Plus, people like things when they're subversive or especially unexpected, and I can totally see people wanting to potentially write revenge-porn based on dosing character/s (not even necessarily MALES) and then knocking them up in the ass (which wouldn't work for sterilex-takers, YMMV).
As for "who's gonna play it" more specifically, I'm interested in preg content of all types, to be quite honest, and regardless, is "it's not going to be super popular" at all a good reason NOT to include a fetish in... a game about weird and niche fetishes? There's no reason someone would NOT want to see this content just because they weren't specifically into mpreg content, and besides! Even if someone was specifically REALLY NOT A FAN of mpreg, personally, they could just... ignore the tf, and not have it in the game at all.
Also, to head off the invariable question: If a writer HAPPENED to be highly against writing a character who would RISK impregnating a character anally, they could easily have the character take sterilex daily and refuse all anal penetration when convinced to go off of it.