Theoretically, women are A/B/O also and omega women can become pregnant too, but no one ever writes about them. A/B/O bears a slight resemblances to BDSM/Directedverse (named after the fic "Take Clothes Off As Directed"), where everyone is born top or sub (I know, those aren't the terms that are usually paired together), it supposedly determines or is related to personality, everyone's bisexual (I think), and only top/sub pairings are "normal" and other pairings (top/top and sub/sub) and switches are not. It's a way of importing traditional gender dynamics into a same sex relationship in order to comment on them. One of the earliest ones has Colonel John Sheppard from Stargate: Atlantis as a sub, and everyone's astonished that a sub is capable of military command, that kind of thing.