I don't like either D/S or A/B/O AUs, but I don't think they're trying to justify anything. They seem evenly split between 'here's a specific unrealistic porn scenario' and 'here's a deconstruction of an unrealistic porn scenario used to criticize real world gender roles.' It's worth remembering that the people writing these things are women, i.e., the same people who have that biological essentialism and barefoot and pregnant mindset imposed on them, and writing ridiculous porn based on those things doesn't imply support in real life.