I know some monos who prefer to date non-monos because they just don't have the time to commit to a "full-time" relationship. They might be busy with work or kids or an ailing mother, and they like the idea of "being with someone" without the "burden" of having to always be available for that person. I also know some polys who prefer to date as "secondaries" for the same reason.
When you get to a certain age, you start to find that most people who are still single and actively dating are looking to settle down, get married, and make babies. Or worse, they're divorced and looking for replacement parents for their kids. If you want no part of that, it can be really hard to find someone who will accept "less" without calling you immature or selfish.
I don't need labels to define me. They're sticky and I hate the glue they leave behind.