Daughter says she is moving in with your boyfriend

Hi, what's the right thing to say to a daughter that has told you that she is planning to move in with her boyfriend. She will be graduating college in May. I told her that it's not the right thing to do as a Christian and that she should live on your own to have new experiences as she starts her career. . She said it's a common thing these days.

My wife and I feel bad as we feel like we fell down as Catholic parents. I'm thinking of writing her a letter that explains why it's the wrong decision in a loving way. It put us in a difficult situation because our son is 15 and participants in his church youth group and I don't want him to think this is normal.

Thoughts or suggestions?
Thanks