A relationship matters if 1 bedroom housing is a necessary part of the plan.
We're talking about marriage, which is a very specific thing with a legal definition.
The study is on marriage, true but the same trends are applying to couples in general - so the same conversation applies there as well. Given the likely causes, there doesn't seem to be a compelling reason to look at marriage in isolation.
As paths to affordable cohabitation decline, relationships are declining right along with them.