Foursome

My boyfriend and I have been together for five years and just tonight he told me he wants me to have a foursome with his best-friend and his fiancé.. part of me wants to say hell yeah but part of me is afraid it will ruin our relationship... HELP

UPDATE: I know it’s been awhile.. my bf thought that i told him it was okay when we had our conversation and ended having multiple threesomes with his best friend and his fiancée.. I’m so hurt I don’t even know what to do 😭