And nobody got upset by this decision. Anal pregnancy is such a turn off I'll never understand why most mpreg fictions insist on having that instead of giving the characters a vagina. It's not like mpreg is a super niche kink either considering omegaverse being a whole genre focused around that.
I think the reason for why ABOverse is like that:
When people think of 2 guys having sex, they think of a "dick in the butt". So logically, if you want them to have kids, why not do it through the butt. So most people can not think of guys ABOverse having vaginas since ass does the job well enough if you do not think about it to much.
The logic of "2 guys having sex=butt stuff" may is also a detriment to cboys/male herms somewhat. Since if we go by that line of thinking, can a sex between a cboy and a guy really be considered equal/as arousing as that between two "normal" guys? "Like, if I wanted to see dick on vagina sex, I would watch straight porn" or something like that.
But most of all, I do think that most people do not find guys with vaginas appealing. Or at least, if you asked someone would they rather have sex with same boy if he had a dick or if he had vagina, I think most people would pick the latter. Now, preferences and all that, so I can not blame them.
But maybe the sparse nature to cboys/male herms can also be attributed to that, it is sparse and nothing major instigated the flames for it spread. Afterall, what people find appealing in men is not that well explored as what people find appealing in women. So maybe there is an audience for cboys/male herms but no one really struck cord with it. Same way as, lets say, Bible Blacl and a lot of 90s/early 20s smut did for female lovers.