<human>: Love is about compromising and trust, I completely understand.

A few days ago, my girlfriend ran into her ex-boyfriend a couple of days ago. She dated him 2 years back for around 3 months and broke up because he wanted to have sex with her and she didn't. They haven't talked or seen each other for a while but when they met, they spoke so casually but he quietly slipped into the conversation "so..when are you planning on coming back to me...hahaha joking." 

She wasn't expecting that so she just rolled her eyes and he quickly changed the topic. the conversation ended right after.

I was not there to witness this but her cousin was behind her while they were talking so she thought it was a bad situation to do anything about it. They do not talk or hang out often, but she still has him on her social media and they would text on occasions. (birthdays, holidays, etc...). I told her that I wouldn't mind keeping him there with the little contact they had before but after what he did, I told her that I think that her not talking back to him was disrespectful to me because I felt like her ex just spat on my face and she didn't do anything about it. And I told her that she should step up nicely and message him saying that it was disrespectful to joke like that or what not.

Although, She doesn't want to hurt him by blocking him because he would then think that something is wrong. She doesn't want to talk to him what so ever but she doesn't want to completely remove him from her life even after what he did. 

She keeps telling me that she wants nothing to do with him and that they are nothing more than just acquaintances. I warned her that if she let him joke like that once, he would do it again. And that keeping him in your life would bring you no good but potentially a lot of harm.

Am I wrong/right? If so how should I deal with this since it is bothering me, or how should I talk to her if I am?  Thank you all for your time.

Give me the gist of the text in a nutshell
<bot>: Girlfriend's ex boyfriend jokingly asks her to come back and she doesn't do anything about it. Still keeps him in her life.