should i give my girlfriend a mmf threesome?

I am 24 yes old and have been with my girlfriend who is also 24yrs old for 7 years.we have been together since our senior year in highschool.we took each others virginity and I am the only guy she has ever been with or gave head to.a few weeks ago while n bed together I asked her what her fantasy is n she told me she has as of lately been fantasing about sucking me an another guys dick at the same tyme. And after she sucks us up she wants us to **** her at the same tyme(train)(gangbang)she told me that its just a fantasy and that she doesn't care if it ever happens or not.I told her I would give her what she wants since she is going to give me a mff threesome.well we found a guy from out of town to do it with.we have set it up for this weekend.now that we are getting close to the day I am starting to get nervous n wondering if I should still let it go through.I have read in other forums that girls have a hard tyme separating their feelings from sex.the last thing I want is my girl to b thinking about this other guy all the tyme or have fillings for him.I'm not sure what 2 do.I luv my lady n wanna give her her fantasy.I'm jot gonna lie the thought makes me very horny n turn on.but afrwards I feel nervous n a little worried.I don't want my girl to end up cheating on me.should I give her what she wants or should I call it off??has anyone done this b4 and how did it go/turnout??no negative feedback please.