threesomes etc are most men's fantasies.....some men will never vocalize it. i seriously doubt if you did it that he would leave you for the other person. HOWEVER,
#1 you sound very young.
#2 YOU don't want to do it and are only doing it to make him happy and that is a recipe for disaster. you should only do something that you both want to do.
if you decide to do this anyway, which by the way i think is a very bad idea given the things you have said, make sure you pick someone you will not be seeing around town....find someone who will go away or the aftereffects of your night together will haunt you.
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