We're not really open with our families, but then I think since its about our sex life I don't see why its any of their business.
I think the most irritating assumption we come across is the idea that if we have other sexual partners it must mean that one of us can't satisfy the other sexually. There was only one person that said we were hypocrites to get married if we wanted an 'open' relationship. (I hate the term open relationship, but poly doesn't quite fit either).