It's a good question. Not sure I have a good answer for it... but I'll throw this out there:
"I prefer autonomy-based relationship rather than possessive-style relationships"
And to the jealousy argument, which is usually the pinnacle for monogamists, I like to remind people that jealousy is just another emotion to be managed, like anger. We don't tell people not to drive because they might get angry at the other drivers, we tell them to manage their anger better.
"It is no measure of health to be well adjusted to a profoundly sick society."