I thought it might be that, but my offer to remove any scenes they found problematic was met with:
"We do not allow the presence of underage characters in Adult/18+ content."
And a follow-up email to their Guidelines team made it clear that they were talking about no minors in the game at all under any circumstances. In short, they weren't interested in actually trying to adjust the game to suit their guidelines - they just wanted another adult game dev off their site.