I've been in a similar situation with friends. Actually, that's how I came to find out more about swinging. They were very experienced swingers, so there was none of the physical stuff. They hinted, and then the wife approached my wife. I researched it and talked about it, and decided it wasn't for us. (I liked and enjoyed the folks here so much I stayed on the forum.) We remained fast friends with them.
I think it's important to interpret their attempts as being naive, rather than anything malicious. Something that remains unsaid can be a problem in any relationship. Maybe just get the idea out on the table: "Are y'all swingers?" Just make sure to project that you're cool with their lifestyle choice, and that could open the door to ending the uncomfortable innuendos.