They're two females. One of them is bi, and gets off on having sex with futa, either behind her wife's back or while she watches, while the other is purely lesbian, and tolerates her wife's cheating because she knows she can't give her what futas can. For the most part the other wife would kinda be an npc, and wouldn't get directly involved in the sex, though it could develop into a plotline later on where she finally decided to give it a try for herself.