Getting paid to stay poor and single isn’t an incentive either and that’s the end of the social spectrum where most of the destruction occurs.
Otherwise, it’s a marginal difference for most income brackets.
You’re right, there’s a big difference by class. For upper and middle classes, living together has become one more step in assuring the couple will do well if/when they consummate the marriage, at which point there’s a high likelihood of them staying together, at least until the kids are raised.
For the lower classes it tends to be part of a revolving door lifestyle, whether or not dictated by government benefits.