Another typical reaction I get when I explain poly to people is one that takes a bit more self-assurance to deal with. The whole "phase" thing is fine because it's not directly calling me out - it's just kind of a misinterpretation of how poly actually works for me. This other reaction comes off almost like an attack. It's the, "poly is not in the best interest of the people you're with" reaction.
I'll break it down a bit more and give a direct example. Let's say I've just told a person I'm poly. Generally we go through the whole gamut of the history behind when I decided to be poly, how exactly I structure my relationship, etc. etc. Then the person will say something along the lines of, "doesn't Now-hubby/Frisbee/The German know that eventually you're just going to decide to lie or leave them?" It ties in a bit with the idea of poly being a phase - that at some point I'm going to like...pick a winner and then dump the other two.
It's sort of looking at things from an overcompensation angle. I formulate my relationships on a premise of honesty and agreement, so this must mean that I'm keeping even bigger, scarier secrets from everybody that will be even more emotionally crushing if we ever break up.
Which is not to say that this reaction doesn't hang out in my brain and sometimes get to me. It's a difficult one to deal with, and I'll sometimes find myself asking Now-hubby or Frisbee if they're really sure they want to take the extra effort that being in a poly relationship entails. From my end, I'm willing to negotiate a schedule or discuss arrangements if either of them found themselves in another serious or even casual relationship with somebody. It can get a bit overwhelming, though, that they're all willing to do the same for me. I'm incredibly grateful for that.