Have you met these freinds. Do they send christmas cards, birthday messages. Do you know there names and what is going on in their lives. If you have been married 18 years I would expect that you would have heard about these people before? Both you and your husband should feel comfortable talking about these people. Actually I would expect an introduction.
It's understandable that you might be cautious as your father cheated on your mother.
Get really clear...is it insecurity on your part...or something to do with his behaviour or change in behaviour.
I caught a cheating partner out after reading this book.
Amazon.com: Never Be Lied to Again : How to Get the Truth in 5 Minutes or Less in Any Conversation or Situation: David J. Lieberman: Books