I was discussing this with my fiancé last night, as we were talking about sister wives and other third parties in relationships. She said "I would always be weary of their intentions." and for good reason I think. I tried to think of some sound reasons a woman would become open to a established couple.

We theorized that sexual desperation could serve as a motive for joining this arrangement, but with the availability of a hitachi would render this null and void. (Or maybe not, idk)

Also given was just the highly experimental nature of young adults.

But besides those two can anyone offer any insight to why a woman would casually (not roomates) join a couple?