But as the roommate brought up, she’s not the only one paying the bills. Yes, she gets some veto power over who can and can’t stay in the apartment, but it sounds like she’s never given any reason why not, not even a simple “I’m not comfortable with overnight guests”. It’s coming off as saying no just because she can, at least to the roommate it is.
No, not every little thing needs an explanation, but this is not a little thing. This is a boundary that potentially interferes with the roommate’s right to have guests in a shared space. Boundaries like that need explanation, even if just a simple one. This is also clearly causing issues between them and needs to be discussed to avoid a hostile living arrangement.
Also, having guests and having a boyfriend over are two completely different things. If it was like, her mom, I could see the issue. But only one of those “guest” has the potential to stay overnight regularly, use your shower, eat your food, and be part of the cause of very uncomfortable noises at night. It’s just a whole can of worms that I understand wanting to shut down completely.