Oh wow...there's a lot to unpack here.

First, LOL @ the Bernie Bro being a small-penis cuck.

Lols aside, it sounds as though he isn't truly into the concept of polyamory (a situation like that requires all parties to have healthy communication and control over their jealousy) and is only going along with it to please his girlfriend and be "cool" for being sex positive. That's a big yikes.