How would such opt-in be implemented? as a separate section or as a special tag? A section might be easier for the permissions, but the tag would make it more convenient for the various types of players and roleplays that might feature such themes.

If going with tag, it might be more appropriate to have several degrees of it, though that could also be covered by GMs. Some might want to just have "a little bit extra", while others might want detailed orgies. Neither of those should be visible to the youngsters (though it is a topic for disagreement where the top age of "youngsters" is, depending on country of origin. Some nations have 18, others 16. Some probably have other numbers again too.