If I remember correctly, you were hired as a cost cutting measure. Shizu breaks things (and people) when she's sexually frustrated and so forcing her to hire someone she would actually bang ended up being cheaper from a legal perspective. Its why Aio puts Shizu in charge of hiring the moral officer. Everyone else is just taking advantage as a side benefit.
Regarding if characters have sex with people outside of the main cast -
I think Aio does - she talks about banging other operators after operations and how meetings with other PMCs turn into orgies. Its never overtly stated that she is still participating, but she still has meetings so one could assume she joins in.
I dont think Adelaide does anymore- I dont have any evidence pointing either way, but it seems like it would be a problem for the organization to have someone so submissive be a senior operator if she was bound to sleep around. You dont want to give your competition that kind of ammo.
Kyva does, she has a line talking about all the "practice" she gets at the front desk.
I dont think Shizu sleeps with any dudes... She used to but I think it got too emotionally draining since they kept dying on her. She sleeps with at least one female director, as a means of obtaining financial support, but outside of that I dont think she does.
That being said, it would be hard to categorize any of this as NTR since they all have pre existing sexual relationships with other characters that continue even after your introduction to the story. Also, all the relationships are very casual, except maybe Shizu.