I currently have a fiance (we consider ourselves married, but haven't gone through with the legal dealings for personal reasons) and a Dom (friends with multiple benefits ;-D)
The hubby and I have had a couple ffm threesomes with friends in the past, but this would be the first time in our relationship that there was a secondary relationship, involving only one of us. It was a shaky start, but he's been really accepting of this and we work through the tough feelings as best we can, which isn't always graceful or pretty.
I had the advantage of starting my solo mission before he did, so when he found someone to spend some extra-curricular time with, I could handle my jealousy and insecurities much better.
And once he did begin his other relationship, his own doubts and such were squashed.
For me, the relationship with my Dom in no way lessened or changed the feelings I have for my husband. And he saw the same reality for himself when he started dating.
My opinion so far is that poly is pretty awesome...now if I can convince the hubby for a mfm threeway, I'd be set! Hey, I'm not too proud to say I'd like to lay back and have a couple of people taking care of me sexually ;-P.