Wife is getting extras from younger swinger

MY wife and I have been going swinging for a couple of years but then I caught her having sex in a younger guy’s car.
My wife and I are in our early forties. We became friendly with some neighbours the same age and one night we got talking about sex. They said swinging was fun and we should try it.
I could not get my head round the idea at first but I asked my wife how she felt. She said she was willing to try it if I wanted to.
We went to one of our friends’ regular ‘parties’. Everyone seemed relaxed and happy and after a few drinks the idea of swinging and swapping didn’t seem so weird.
We played various games to decide who went with whom and there were lots of laughs. No one saw any of it as being a problem. Before long we were doing it regularly. It even seemed to improve our own love life which had become a bit stale.
Then one night I was supposed to be working away but there was a change of plan. I drove home late without bothering to phone my wife. It was after dark but my headlights picked out a car I recognised parked near the recreation ground and I thought I glimpsed my wife’s blonde hair, so I turned round and drove past again.
To my horror I realised she was with a younger guy who had shown a lot of interest in her at the last party. They were hard at it and oblivious to everything around.
My wife looked surprised and shocked when she arrived home to find I was already there, but I didn’t say anything and I don’t know what to do.