Friends with exes

I'm curious to know what people on this sub think about being friends with exes, and particularly exes with whom grave sin (premarital carnal relations) were committed.

In the secular dating world it seems to be the norm and if you aren't ok with it people make you out to be uptight, closed minded or controlling.

It has never sit right with me that a woman I am in a relationship with is friends with someone she has slept with before. And by friends I mean communicating regularly, has pictures with on social media, planning social engagements together, talking on the phone, etc.

It has created unnecessary pain and ultimately undermined relationships that could have otherwise turned to marriage. Not necessarily because she fell into sin (as I have before too), but because she chose to make that sin an ongoing part of her social life, and ultimately of our life together.

This recently happened to me with my girlfriend and her female friend. The homosexual component made it extra confusing and hurtful for me.

What do you see as the Catholic approach to dealing with this scenario?