I'm gonna wager that's not the reason.
If we stick with the thread of some VinDad headcanons, it's possible Hojo was shooting blanks.
I'm probably posting too much here XD sorry all. That said,
should Rebirth establish that Vincent is Seph's dad, I'd be one happy camper. What a twist years and years in the making. I'd be totally satisfied, but I doubt that'll be the case.
But even so, if Hojo can't pop out a baby himself, I still suspect he'd breed Ifalna. The game does even directly talk about this. In Remake and the OG even, he plans on breeding Aerith to not lose another Cetra. So I guess he was satisfied experimenting on Ifalna and that Aerith (a half cetran) was a fine backup and seen as just useless in comparison. And I'd probably be sated with
that part of it, but I'll still be left deeply unsatisifed with the notions of Hojo not experimenting with Ifalna's cells and Jenova cells in some sort of freakish chimeric being. (Maybe that's the Heretic transformation for Rayleigh and Hojo, again, satisfied and done). Maybe it'll be the Psychic P kid from CC (maybe that'll be Chadley?). I'm not sure. I just hope somewhere, at some point, we get some sort of consideration.
I figure some of the experimetns had to drain Ifalna of blood and keep it in storage or something. After all, Hojo and Shinra are willing to wait for Aerith for some time. There really should be a deep expalantion for this.