Meg Jay, a clinical psychologist, has a fine piece in the New York Times on how sliding into cohabitation can make it less likely that you will make a successful marriage.
This line is worth the price of admission:
"I’ve had other clients who also wish they hadn’t sunk years of their 20s
into relationships that would have lasted only months had they not been
My advice to students: don't live together without a ring and a date.