Just because I don't feel pressured to find more partners, doesn't mean I'm closed to the possibility. If I was, I would have said that.
Boyfriend number 2 didn't assume that I was "all set" since I already had two partners, he asked questions about my availability and what I wanted in my relationships, and now we're a couple.
You raise interesting questions about positive versus negative intent. Basically, unless I know for a fact that the person is poly, I assume he's just trying to get laid, which doesn't generally interest me. I don't know if the person who wrote to me identifies as poly. He's certainly nonmonogamous and that's all I know. He's far nicer than the run of the mill bootycalling creeps who usually write to me, but that isn't saying much. I would have asked him more about his relationship styles if he didn't turn me off early on.