Honestly? By all rights, AO3 shouldn’t be hosting any adult content, because they aren’t an age-restricted site, and have no method in place to verify the age of a reader. What they DO have is a seriously deep-pocket not-for-profit backing them, and a team of lawyers who are exceedingly adept at obfuscation.
I’m not saying anyone shouldn’t post there, but I will venture my opinion that if a legal issue arises over adult content, AO3 will throw the author—I’m sorry, “creator”—under the bus faster than a cartoon villain.