I have been in a similar situation twice. One time it was because my girlfriend was becoming more emotionally distant from me. So she hoped I would find someone else, so she could feel better about leaving me.
The other time, it was because I was working out of state for a few months and my girlfriend thought it would be good if I still had regular sex. However, when I did find someone, she became very jealous and wanted me to stop. She wasn't as ready for it as she thought she was. I think some of her wanting me to be with someone else was because she cheated on me and wanted me to do something to call it "even." (I didn't guess at this until much later when I figured out a few things.)
Not sure if that helps.