Shared household is enough of a criterion. Noone would force roommates to get a civil union if they don't want/need it. Just like noone forces a couple to get married under the current jurisdiction.
I'm not sure that cohabitation is the best criterion.
There would, of course, need to be exceptions for deployed military and those with jobs that take them away from home (thinking truckers, folks on fishing boats or oil rigs, etc.). I'm guessing one's legal residence would count toward cohabitation there, even if they don't spend much time there.
However, that still excludes spouses who are separated for some length of time, either by having to work overseas or in another state (and establishing legal residency there). Cohabitation currently isn't a requirement for marriage, and establishing it as such would disenfranchise a lot of folks in established marriages.
Just pokin' holes with my pokin' stick. Nothing personal. I like the discussion.