I’d say they should. I think there’s a differerence between civil marriage where the definition of marriage is redefined and civil union. But I’m only ok with civil union in the context of two platonic friends (same or opposite sex) not the assumption of civil marriage which is where two people behave inappropriately.
In other words, I don’t think we need to redefine marriage to mean a union between a man and a woman but we do need to redefine a civil union to mean two (or more) close friends who share a common association and agree to live as roommates but to have a prescribed legal remedy both for getting together and dissolution. A civil union should have the same rights and privileges of marriage until someone marries an opposite sex partner which supercedes the civil union and results in property and rights division (similar to civil divorce). Even though sexual intimacy within a civil union is an immoral result, the ability for people to share resources and rights seems like a greater good. My greatest issue with a civil union would be if it became an impediment to traditional marriage which is the union between a man and a woman for the intent of the procreation of children.
I am a married man. I think marriage is the union between a man and a woman meant for the procreation of children. I feel a civil union on the other hand is an agreement between two people to share resources and other tenative benefits typically associated with marriage, such as being a next of kin, marital confidentiality in court, an ability to get spousal support, and an ability to share property.