Wife Swapping

Hi, I’m a 38 year old woman, I work as a teacher & my husband works into corporate. We have a kid who is 8 years old. Life is going good. We both are very passionate about our life, kids & traveling. Two years back my husband made a new friend who works in a different company and they are working together on a project. This guy is also married with a very beautiful wife. We all went on weekend trips along with our kids thrice so far. We enjoyed eachothers company. A month back we went to Maldives for a week, We thoroughly enjoyed our trip but something very weird happened on our second last day that I can’t take it out of my mind. It is still stuck. My husband asked me to sleep with his friend so that he can sleep with his wife under influence of alcohol & he laughed after that. I didn’t react much because lot of thoughts started creeping up, I just slept that day. Next day I wanted to discuss this my husband, he seemed to have forgotten the discussion. I tried to make him remember, he is laughing it out that he might have said it in a fun way not something he really wants. I still cant believe my ears & he is taking it very lightly. What should I do?