They are absolutely not sex clubs. They are clubs for people in the swinging lifestyle to meet, play, and get to know each other. IMHO Sex clubs are where you pay for sex with someone.
No, they aren't always super glamorous, and yes, you will find a complete cross-section of the human race in them, but most of the people in them have been swingers for years, and enjoy the frisson that sharing themselves with someone that isn't their partner a huge turn on, and a part of their sex life.
Yes, on "single guys" night, you may be hassled, and followed around by guys who are just there for a shag, (remember, no means no!) but equally, there are single swingers that are decent, genuine, intelligent and articulate. Sorting the wheat from the chaff is the hard part!
You may not see anyone that takes your fancy, you may go on a night where there are three other people there, you may get really lucky and find lifelong friends.
Go with no expectations, relax, have a drink, chat, and see where the evening takes you.