There's a Plenty of Fish or OK Cupid survey somewhere where heterosexual female respondents reported sex within the last year at a much higher rate than heterosexual men. The implication is that some of the more attractive men are having sex with more than one woman. There are some other surveys out there

In my personal experience, the average person having sex with more than one partner is unlikely to identify with the term polyamory. It's not as mainstream as the internet makes it seem.