Ah, that's reasonable. One issue with with the census questions is that they're mainly focused on defining a "household" and ignore situations like being in a r

Ah, that's reasonable. One issue with with the census questions is that they're mainly focused on defining a "household" and ignore situations like being in a relationship but living separately. But married + cohabitating should give a good first-order approximation of the non-single population overall. Unfortunately, non-scholarly articles about dating pretty much always use the legal definition of "single", probably because deciding who is single in the dating sense is so tricky.