I've been on both fences of this argument.. But really, generally, it is OK provided the 18+ content is done behind closed doors, e.g., I think it's best to advertise a venue as mature to steer people away, than it is to let someone play the whole "Why don't you mess around and find out game". It's arguably just as safe on both the 'child' and the people at the venue.

and frankly policing what people do behind closed doors is a pretty bad precedent to set.. First and foremost because you will very quickly escalate how people feel about all things against ToS. Should they start banning people for simply showcasing a mod regardless of whether said mod is harmless or not when done for personal consumption, e.g., parsing or character mods... Shall we ban those people too? -- Shall we then start banning people for simple talking about them? Because there absolutely is a clause in the ToS that stipulates "- Creating, distributing, using, or promoting utilities that interact with the game."

Should we also start banning people for profanities, because many people absolutely do find it deplorable to express certain profanities in the presence of a child.

This may come across as pedantic, but this absolutely will happen.