Tricky question. Of course, it's easily turned around. Is my love not enough for you? You need exclusivity as well?
Anywho, Vonnegut wrote something fantastic on the subject..... *shuffles through library* It's from a speech he did, for graduating students at some college.
It's stuck with me.
"Let me beguile you just a little bit more about extended families. Let us talk about divorce, and the fact that one out of every three of us here has been or will be divorced. When we do it, we will very likely wrangle and wail and weep formlessly about money and sex, about treachery, about outgrowing one another, about how close love is to hate, and so on. Nobody ever gets anywhere near close to the truth, which is this: The nuclear family doesn't provide nearly enough companionship.
"I am going to write a play about the breakup of a marriage, and at the end of the play I am going to have a character say what people should say to each other in real life at the end of a marriage: 'I'm sorry. You, being human, need a hundred affectionate and like-minded companions. I'm only one person. I've tried, but I could never be a hundred people to you. You've tried, but you could never be a hundred people to me. Too bad. Good-bye.'
This was in 1974, and I'm sure it's more than 1/3 now. This is the best explaination to why my previos long-term relationship ended, and a large part of why I choose to live as a poly now.
It's not pretty words, not that useful in winning anyone over to your side. I tend to get "you'll always be enough for me", and never do I believe it. But it helps me to see why I live as I do, why it's right for me. Which helps me explain it to others.