questionable relationship activity?

I have this girl friend whom we have a view for marriage later on. But I am worried about what she does on her smart phone. You see she was once on very active on an adult for sex site. And though in eight months, the entire span of our relationship, I know she still talks to her contacts from that site. I have dared to pick up her phone and look for my self a couple of times and always its distraught me. Things like "are you up for some play time?"; her reply would be, not right now I have something good going... OR "when are you going to send me some nudes?"; her reply would be, "I don't have any on my phone". Its clear that she wants to be passive in redirecting these people without putting them off.

In the past, when I've attempted talk about how I feel on certain aspects of our relationship. She does get defensive and bashes me most sternly with the obvious valid points. In this case, she would just say that she hasn't done anything with them. And then make the issue about trust until your left with nothing but the obvious stay or go situation. Which isn't a fair position because I want her, I just don't want that in the back ground of our relationship.

Thoughts please... I'm 38: she's 43. I don't understand this, relationship or not, why would someone not close the door on this life style so late in life and is this something I should really be concerned?