The civil union idea was around for a long time. But the original “civil union” proposal was meant to get a household of two people (including those such as an adult child living with and caring for an elderly parent, siblings living together, or just two people - who may or may not have been homosexuals or could have been a heterosexual couple in a “common law marriage” or even long-term roommates. This would have given them tax breaks and other perks, but was not the same as marriage and did not require a sexual relationship. I think it would have been fine, and I think the “civil union” concept should revert to this one and not be sexually based.
I don't think it is fine. Tax breaks were meant for married couples raising children. This crap about siblings, friends, etc is a farce. What if one has more than one sibling or both parents are still living?
You're argument leads to legalized civil polygamy.
Pretty soon nobody will be paying their "fair"* share of taxes.
*N.B. I don't think the current progressive tax system is fair.