@mietze The ICK factor for me is... NPCs are different than PCs. Usually that means they have some plot-purpose for the wider game or they may have different stats or something.
If a staff-player wants to get some TS on with their perfectly normal PC alt (or just have some light chit chat over coffee or whatever else to get a break), that's a-ok. Not everybody can run big meaty plot scenes with all their time and needing to indulge their own personal interests is normal, should be no shame in that.
But staff breaking out the Vampire Prince to go have sexy times with a specific player or whatever else... that's gonna make a lot of people uncomfortable and best just avoided. If it does become crucial to seduce a powerful NPC, just FTB and move on.