It happens more then you think and that's why a lot of couples either travel long distances to meet up or specify a one don't have that problem as we talk about everything (sexual ) and always have .The more you participate with different people the more chance but the gains are better then the downfall it just depends on what your looking for .Ex A couple that gets to know all about you kinks ,special spots or sensitive  locations over the long term and the comfort that goes with that or the rawness of new people in the mix all the time and more or less you start off from scratch every time you have to term can lead to jealousy too.

rusty