You do realize that people settle all the time right? The ex broke up with her and it destroyed her to the point where she almost wasnt ready to start a new relationship when OP met her. Now 5 years later she is still talking about how wonder the ex was to her friends and literally saying that her current partner could never measure up to the ex. She got dumped by her plan A and OP became her plan B. Nobody wants to marry someone who sees them as a plan B.

So OP now has to think about how much his partner is pining for her ex every time they have sex, because that is after all the aspect in which he will never measure up. So every time they have sex is just a reminder that she settled and that she isnt over the ex that broke her heart all those years ago.

Talking about relationship issue with a friend is very different than straight up saying to a friend "my ex fucked me better than my future husband ever has or will ever be able to". That is talking about a relationship problem, that is shit talking your fiance and reminiscing about how great the one that got away was.