Not necessarily. I have seen people who have no sexual relationship whatsoever, who could benefit from civil union to save on taxes etc.
One example I know of were a couple of brothers, both born with a genetic condition that pretty much excluded them from the hope of ever attracting a woman or hopes of marriage or family. Both decent guys, but both also psychologically and socially impacted by their conditions. (They were tough to look at.)
They got older, and the only family they had was each other. They lived together in the same house that they bought. Then, the older brother (who did work at a decent job and had a few assets) died. His brother and only kin, and the only other person ever close to him in his life, had to pay taxes on his estate. Civil Union would have prevented that outrage.
Kill the Income Tax and the inheritance tax, and there is no need for Civil Union.
Toss the inheritance tax - a/k/a stealing money given from one family member to another - and let people establish private interpersonal contracts which are valid only between the signers.
The peril of civil unions is that forced recognition / subsidizing of them falls on everyone else.