Help Hubble want to add a man in the mix

Can someone please give me an opinion my partner has ask if he can invite a guy over to watch me suck his cock while he fcks me. I want to but reluctant because I was already feeling before he mentioned the mix wether he was still attracted to me still and worried the effect it will have. we have been together 12years and have struggled to conceive for 7yrs which is straining the relationship too. Please someone applied who been here before or has a non judgemental opinion. The tension from him could cut a knife.