My boyfriend is having gay sex

My boyfriend and i have been together for just over a year and a half, I've cheated on him and hes cheated on me but we are so happy together and it was a long time ago at the begining of our realtionship. Now my bestfriend is my boyfriends bestfriend as well but they have been getting closer and closer until they were foreplaying at first i didnt mind because it make my boyfriend happy so i was all down for it and down for him making other people feel the same way he makes me feel but ive been a bit iffy about it lately as we just came clear about all the cheating that has gone on and my beatfriend keeps telling my boyfriend thibgs ive done in order for him to have a go at me now last night my boyfriend was round my bestfriends and he told me they had sex and im really not happy about it, i dont want to take away what makes him happy as my boyfriend  has depression and its hard to keep him happy but im really not happy about it what should i do?