I mean honestly should be in the rules that anyone engaging into Erotic Role-play should ask oocly via DM in game if the other person or persons are 18+ and consent to the role-play. Now if someone lies about their ages by claiming they are above 18 then that's another story, a bit like going to a bar and hooking up with someone that's 17 but had a fake ID that said 21... Obviously illegal and disgusting to have ERP with a minor, no exceptions. And no one under 18 in game should do any ERP or have any kind of sexual oriented role-play or behaviour.
Or add a system that proves your age but even then that's a lot of efforts for something that'll get breached.
I'm not sure what's the point of role-playing full ERP when you can just fade to black, I personally don't see the purpose, maybe just role-play after the act, not sure on how engaging in a fully detailed sexual encounter vs faded to black adds anything but some people want it I guess?
Also what's worse? Detailed erotic role-play of two adults having sex or detailed gore torture/murder? I think the latter is much worse...