Gabi has one for us, and I think I know this one too!!! But I’m not 100% sure because I only read book #1 and I have to finish the trilogy, but by the reader reactions (and HUGE love – and some that were fuming) I have a feeling this is it!!
Book where two couples decided to switch spouses. One couple was more into it than the other and started an affair after the couples had decided to stop.