I might be missing something here. The OP mentions one acquaintance is interested in going with him, and the other, who the OP has invited to the beach, has shown no reaction. Not sure if there are some undertones in either case. If not, then I'd let it go unless he welcomes to company of the first, or until the seocond brings it up again. If yes, then he might purse from a differnt angle, although the guy sounds not interested.
I was in a very similar situation years ago. A former buddy told me after years of friendship that he went to a local nude beach, and subsequently asked me a few times to join him. He was still married, but in the early stages of a divorce. The nude beach invite came out following while his volunteering he had done some experimenting in college, and was thinking about pursuing it after his divorce. I was not aware, and not sure if it was part of the divorce, but happy that he felt comfortable sharing such details. I was not interested in joining him at the beach, maybe given some of what he shared. We remained close friends for a couple of years until he met someone and our friendship cooled. Great guy and friend, and miss him.