Is it normal for men to be propositioned by swingers, and what makes a man an attractive candidate to swinger couples?

I have been propositioned by swingers quite a bit. It's to the point that if a couple is nice to me, I start getting suspicious. This has happened with acquaintances, long-time friends, people I've met at a party, co-workers, and even strangers at the beach. Three times that I can remember off the top of my head, they propositioned me and my girlfriend at the time (separate girls many years apart). Is this a normal thing? What about me could make me attractive to swinger couples?

I'm a cis-gendered straight male. As far as physical attractiveness, I would say I'm slightly above average based on what women have told me and how they treat me.