I have a partner who is bi, and he has a bf that he sees from time to time. It is also more of a sexual attraction than a deep relationship. Altho, with the guy he is with now I have seen that it is developing to more than a friendship. Which is perfectly fine with me, but having said that, I recently had the opportunity to join them. I have been with him and other guys before, but this was different. I really felt like I was intruding and didn't belong there. And I think in the future, at least for a while, that I'll find somewhere else to be when they are together. I am really happy that he has found such a friend w benefits, but I don't want to intrude on that personal time for them. I also have another relationship, but it is long term and much more than just sexual so its quite different than what they have.
All relationships are different and its just whatever works best for you. But I wouldn't choose to be the odd one out if my so was involved in the bedroom w/o me.