It will always be the case that people do know know what they do not know. In other words, sometimes a problem will crop up that one never knew was a problem until after it happened.
While it may not have been the case here, with newbies in particular, people never truely know how they will feel about seeing their partner play with someone else until it actually happens. It's one of the reasons a number of experienced couples will avoid newbies, and there is merit to that.