Maybe I am mistaken but it seems like this thread is leaving room for woman to woman sexual intimacy between sister wives. Romans 1 is clear that any type of homosexuality male or female is considered sin.
Romans 1:26-27
26 For this cause God gave them up unto vile affections: for even their women did change the natural use into that which is against nature:
27 And likewise also the men, leaving the natural use of the woman, burned in their lust toward another; men with men working that which is unseemly, and receiving in themselves that recompence of their error which was meet.
Verse 26 is warning against female homosexuality. If v.26 does not stand out in your mind as being female homosexuality, the first words of v.27, "and likewise" tied to male homosexuality should help with context. The Word of God is warning against homosexuality for both male and females.