Well, you kinda opened the door for that to happen by discussing and pondering the other couple/sex consideration. If you truly feel so uncomfortable about sharing your g/f with anyone else, you should have never brought that conversation into your relationship at all.
Buuuuuull
shiiiiitttt
Expressing fantasy is completely normal and healthy in a healthy relationship, and no-one should feel afraid doing so could jeopardize it.
Then there is always the health considerations.
Recent TV commercials have let it be know that one in five has genital herpes.
Not something I want to live with for the balance of my life.
Something to think about to be certain. I wish you well!!!