My girlfriend is talking to her ex about sex. What should I do?

[Nalaban]

My girlfriend logs into her facebook on my computer sometimes. Well, the computer saved her login information and logged me into her account by default last night when I logged on. I saw that she had been chatting with her ex for several hours late into the night. They were talking about how good the sex they used to have was, and he told her that he was going to miss her mouth. She told him that she would give anything to be back in the days when they used to hang out. She asked him what he was going to do without a hot girl to help him pass the time.

What should I do about this? I know I was wrong to read the conversation, and that i invaded her privacy. However, what I found is extremely disturbing. We've been together for over a year and she tells me every day that i am her dream guy and the only one she wants

How do I reconcile this? Should I confront her, tell her that I invaded her privacy, and tell her this is inappropriate? Or should I just lay low on this one?