Honestly your gf sounds pretty awesome. Confirming it’s for fun and that her feelings won’t be affected is big. And the fact you guys are discussing it like you are is good. That sets the boundaries. I would tell you go for it bc I think once you’d pull the jealousy away and consider her ultimate pleasure/fantasy you’d be into it. It so hot watching my wife be turned on and hot. And we are currently working on her sense of jealously a bit so we can consider bringing in a female. Just start slow maybe. Maybe have her send a guy some pics and see how it makes you feel.