I agree with them. Marriage is an act of the church. Civil unions, regardless of sex, may be a government matter.
When my son told me they were going to the Court to get married, I told them I wasn’t interested. What you pledge before God matters to me. Anything else is just a tax shelter.
Why have the government get involved at all on who sleeps with who?
Just allow the legal aspect of a partnership be decided by the state, (Rights of survivorship and who gets what if the partnership is dissolved) and leave the "union" part out of it.
If you want the "union" to be recognized you got to your Church.