Yup, I definitely encounter that mindset. It bothers me a lot.
But for me, there is also the added level of confusion that I like non-serious/ casual relationships (can I call them relationships?) and feel happiest in that type of arrangement.
My interest in poly came out of being involved with two people non-seriously (and later, three) and feeling I didn't want to get serious with either one and then have to give up the other. This arrangement lasted for years. (Then it blew up in my face).
I didn't know, at the time, about the possibility of being serious with more than one person--but also, I didn't feel a "serious" level of compatibility with either guy. One was my boyfriend-turned-ex-boyfriend-turned-FWB, and the other was a FWB. I liked them both immensely and cared for both deeply, but didn't want, say, to live with or marry either of them.
Anyway, I do feel incredibly frustrated when I encounter people who are dismissive of the possibility of non-monogamous relationships. It makes me sad.
But I also encounter a lot of people who do not feel that non-serious or casual dating has value, either. And that hurts me too, because I have grown and continue growing from that type of less-committed dating.
Single, straight, female, solo, non-monogamous.