Can I ask my friend to stop talking about her gf?
Why do you want your friend to stop talking about her girlfriend?
I think that her disclosure of intimate details about her relationship is inappropriate.
It makes sense that you would not be interested in hearing about your friend's girlfriend all of the time. Your friend should understand that she is oversharing details of her relationship when you talk to her.
I feel awful that I pushed my friend away. I didn't know how to react.
If you're that upset then you should reach out to them and see if you can salvage your friendship. Just be honest and tell them that you'd like to set up some boundaries regarding her talking about her sex life. Friendships can still exist without anyone oversharing.