text: Most guys, including me, enjoy seeing two chicks having sex.  We're turned on simply by looking at a naked women - so seeing two women have sex is just simply a bonus.  You should make sure it's something you want to do too.  And if you do then you have to think about jealeousy issues and him joining in.  Of course he's gonna want to join in.  No guy just wants to watch and then go home.  So think about that.  Then if he does join in are you going to get upset if you think he's more into her than you.  There are lots of things going on here that could toss your relationship around a bit.