... is a phrase that can have a perfectly innocuous meaning. I live together with my neighbours, with my work colleagues, with my friends.
It can also mean moving in with an opposite sex partner without getting married. For different couples the circumstances may vary, but I have a sense that it will in most situations be the girl who makes herself most vulnerable in these arrangements. That vulnerability is both material and emotional. [One can perhaps extend the idea of "living together" to same sex partners moving in together, when I sense that the range and dynamics of vulnerability might be much deeper.]
The readiness with which young people do this makes Catholic Analysis' post on the subject of relevance. I think the suggestion in this post that there is something lost that cannot be regained is worthy of particular reflection.