Vanilla person here. Do women actually find it "hot" to be "loaned" like a sex slave? Given the circumstances I don't think it's absurd to assume that this was his fantasy and his only.
In my experience at a college in the northeast, it was about 10% that had actually thought a lot about it already and really wanted to do it. Around 50%, however, were more than happy to try it after learning about it.
This is definitely pretty far on the spectrum (the loaning part more so than the slave part), but I've said the 'loaning' part to people many times in a certain context and never actually meant it. I'd bet it just stayed as a fantasy between them.
Since the comments here seem to be mostly like "well at least some of them do", I'll just go ahead confirm your suspicion as a woman who has talked to other women about their relationships. Liking to be denigrated is not the norm at all, it's kinda gross and demeaning. However, much like many other sexual practices that many women find distasteful, some women will play along or pretend that they're into it to make their partners happy. Most of the time the partners aren't trying to be controlling or whatever, they're just imitating porn which normalizes that kind of thing. So yeah, there's a high chance this was a one-sided fantasy like you suspect.
> woman who has talked to other women about their relationships
That's less relevant as actually having sexual encounters with a woman. People often don't know their close friend's fetishes, but they do (or should) know what their lovers enjoy.
For a counterpoint: open a book of women's fantasies written by and marketed to women.