Nycindie didn't say people who aren't in multi relationships are not "legitimate", whatever that is supposed to mean in this context. "legitimate" means "legalized", so by that definition, nobody's "poly relationship" is "legitimate" since a person is only allowed to be legally married to one other person at a time.
I also interpreted the question asked in the OP the same way as nycindie. They aren't asking "why do you consider yourself poly"; they want to know why a person would "practice" nonmonogamy. To me, this is very similar to referring to "the poly lifestyle". I dislike that expression and tend to avoid responding to posts that i perceive as presenting nonmonogamy as a "project" or an underlying motivation that influences every other aspect of my life. I don't go around like, "may i see the poly menu" "i would like a seat in the non-monogamous section, please" "your server will be the single white monogamous redhead" "would you like a poly drink before you order your asexual meal?" "the poly service was very good, i shall tip 30% on the bill".
To answer your question: yes, i can "be content" with one person emotionally and sexually. I have been, and still am, content with both of my relationships, both people at different points in my life have been the "only" and i was content with either of them as i am with both, and with myself. I am not one of those people who feels like something is missing just because i have only one relationship. And i do not feel "illegitimate" by any means at all.