Perhaps she is holding tight to some cultural and societally taught notions that dads and mums have a certain roll in society that doesn't include other lovers. There are many implications of what we are suppose to be as parents, perhaps these are important for her to uphold right now and she figures you should uphold them too. Perhaps when you first got together and were carefree lovers, she was able to open her mind to more lovers with you.... I wouldn't be surprised if she got back to feeling like that again sometime (like in 3-4 years
). I did.