I find it interesting that a lot of the responses were about sex. When I read the original post I thought about other matters that come up within relationships that one might not want to have shared.
Medical problems, job issues, arguments with a friend, problems paying a bill, etc, etc. Just because my gf tells me about it doesn't mean that I need to run tell my husband. See what I mean? There is a huge host of situations and circumstances that arise in every day life that we share with our primary partners that we might not want their partners to know about and vice versa. In my humble opinion, the original question goes far beyond sex.
For the record, telling me something is pretty much the same as telling my husband. In the future, as I develop more poly relationships, I will be sure to ask my partners if it is acceptable that I tell him about serious issues.
Pinky, 41, f, bi. Married to Bear, 42, m, straight.
Not dating anyone right now.
I became insane with long intervals of horrible sanity.--EAP