Here's a question: why is it that literal cucks (people who are into cuckoldry as a fetish) are always men? Why aren't there women who get sexual excitement out of seeing their men get fucked by other women?
@alyx And since @yuri gave up on any argumentation, I think there still is the equvivalent term. The problem is, that it is so common, most people would simply gloss over it.
but "side bitch" or "side chick" seems to be the closest equivalent.