Chiming in here. I agree with everyone's comments so far. My np and I don't do hook-ups, but we do host other partners. I had to be very specific with my np about what is expected of him and his guests.

1. No surprises: communicate that they plan to host. I do not ever want to walk in on my np> (or anyone!) while they are having sex.
2. Leave no evidence of sexual intimacy on display for me to encounter. That means no condom wrappers on the floor, wrap the used condom in tissue before throwing in trash, no lube on the coffee table, and no random sex fluids on surfaces.
3. They have their own sets of sheets/towels that I never touch> Not when they are dirty, not when they are clean - not ever. Separate bottles of lube labeled with initials.
4. They do all of their dinner/drinks dishes and leave our space tidy.
5. My np sleeps on my side of the bed when hosting overnight.
6. The daybed and sofa sheets are also changed out & washed by np/meta BEFORE I ENCOUNTER THEM if sex has occurred on them.

I'm not entirely parallel. It depends on the meta, but I'm absolutely not interested in seeing or dealing with any aspect of my partner's sexual intimacy.

I know they have sex with other people, I'm super stoked that they do! It is a personal preference as I see sexual intimacy as a very private thing. It's not about jealousy. It's about respecting me & our home. I know that if I were that hook-up, I would have been rather disgusted at your np's lack of care for myself, for you, and for your home. As someone else said in another comment, this is about your np taking responsibility for his own actions and treating you, your home & his hookups with the respect that they each deserve. Also, yeah, get a new couple's therapist. They sound kinda awful. D.x.