My boyfriend is monogamous to me and we live together with my husband and kids. I generally sleep in the room with my husband, and have mornings with my boyfriend. It's clearly a secondary relationship-although my feelings are primary for both of them.
He's very happy and very satisfied. He is free to have other lovers and even a girlfriend who could be live-in if it went that way. But, he's satisfied and says he simply doesn't have time to pursue another relationship.
So, to answer your question, yes it can be fully satisfying. It simply depends upon what exactly is available and what exactly is wanted-whether or not they match.
"Love As Thou Wilt"