Well the couple is 40+. I doubt they'll be attending a college party and banging young studs regularly. They aren't looking for people to bang either. They are mostly experimenting with people they are already close to, already know or have some attachment to. Arnold was their closest friend so he was an obvious choice. Eric being a co-worker helped him get in the lil game they play. The grandkids got in by being related to Arnold. The old spa dudes, are there from making friends with Arnold. The boss got in there as again, a co-worker.
They only do stuff with people that they like being around, and they've both said that they don't like Mario. So he'll never get any.
Hmmm I think only the little guys on the bus was the only person to get any action without some relationship with another character.