Is girlfriend being fair?

Been dating my girl for five years, have a fairly happy relationship. Stated talking about involving others in our sex life about a year ago, usually during foreplay....it makes her really wet when I talk naughty.... had a threesome with her and my brother a couple of months ago, thought it safe and it was a good time, lived seeing her being played by another..gave her a Hall pass to play with another girl a month before that....it was all good and I felt no jealousy.  Now the opportunity has presented itself for me to live out a fantasy of having a mmf with an old co-worker who is 20 years old, petite and down for me and my friend to pleasure her.  My gf is freaking out and is passed that I would even consider it...said she is Grosse out by the thought of me even wanting to b involved. Is she being fair, or selfish. She does have selfish tendencies in other aspects of Inuit relationship. Would like some feedback from others.