Technically it's not an open relationship. It's one where the BF is dating a sex worker, so has to accept her clients. She also told him at the start that if he couldn't fulfill any of her fetishes, she would find someone who would (whatever that means)
The issue is that some of her clients are very close to her. I know she has one regular client who's been seeing her for two years, this guy recently helped out a lot renovating her BDSM studio and they were growing closer. She also uses him for Onlyfans filming. There's also at least one other regular client that she counts as being very close to. And both these guys are effectively her "slaves"
So yeah, it's like dating a sex worker/porn star. Except she's not having sex on camera, just doing fetish stuff eg whipping, strapon etc