Run
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Tell her no. If you don’t want a threesome then don’t have a threesome. If she leaves you over it then chances are she wasn’t the one for you. Threesomes can have very negative impacts on relationships if the attention during it is unequal or focused between the two people who aren’t together, even more so when it’s with a friend rather than a total stranger whom you’d probably never see again. If you want to do the threesome make sure rules and boundaries are set, if they get broken at any time you have every right to stop what’s happening.