Originally Posted by MissBehaviour
I don't personally think I could assume non monogamy as a starting point for a new relationship - not past the dating stage.
See, that's the thing...the way you're talking he still IS in dating stage. But let's say that's not so, that you're truly in a relationship.
1) You and he haven't set up rules yet (at least you haven't said so).
2) It's already non-monogamous, as you have another partner.
So, for example, say I start dating this guy and he has a girlfriend. I start dating another guy at the same time. The first guy gets upset. I'd be annoyed that he expects me to be monogamous with him; when he isn't with me. Regardless of where in the relationship we were. Exception would be IF he and I had talked and we had decided to do a "polyship" together, where he and I decided we'd both date the same people. If, at that point, I started a new relationship, without his having any say, I'd have broken our arrangement.
If you want your relationship to be similar to the above, I think that's perfectly acceptable. But I do think you owe him the courtesy of letting him know your expectations before you get upset with him.