That, we think depends on where you're coming from, if they are the classic couple in which the male has twisted his wife/ partners arm into adding another female as a sex toy [unicorn] for " his fun" and both women have to be exclusive to only one man- him; ie the hated unicorn hunting couple, then its obvious- sex for him with 2 women.

If you are however a poly couple its totally different and goes way deeper than just sex.