My opinion (Just an opinion)
Could care less who staff is tsing or romancing on their pcs so long as they not pressuring anyone or being a jerkface about it. It is not even my business and for me to spend time worrying about that would feel creepy af to me.
I also support staff in having pcs who engage in romance/sex if they so wish, because it is important for us to still have staff on games and if we put too many restrictions on them we are not going to have many willing to give up their own fun to staff.
I don't really care about staff doing ts/romance on minor npcs who don't have a lot of power/influence/sway or as someone said before in sandbox games or slice of life type of games. It also is just not by beeswax what people are rping privately.
However when tinysex/romance between powerful npcs and pcs is starting to have massive and noticeable effects on the game's ic storyline and ooc culture, it likely is a problem.
If pcs are getting huge plot advantages or lots of gear/wealth/rare items, an edge in PvP conflicts, having catty fights over npc's sexual attentions, toxically making sure to tell everyone they are tsing staff so you better do what they want and etc, then it is a problem.
Nobody should feel like they need to ts with staff to get ahead, get plot, get stuff on a game.
But the biggest thing to watch out for staff (on npcs or pcs) tsing with players is that nobody feels likes they will have negative consequences for not doing this rp.
Even good intentions can go haywire and perception does effect things even if nothing was mal intended. Also the staffer doesn't need to be toxic for there to be a problem. A toxic player might take a lot of advantage of it being known they are in a relationship with a powerful npc.