Your sentiment is one that I’ve seen many times before. I understand and respect the sentiment behind it, but I wholeheartedly disagree. Here’s why.
First, the Church makes no distinction between “civil unions” and “marriage”. Just look at the Vatican Congregations for the Doctrine of Faith’s document
Considerations regarding proposals to give legal recognition to unions between homosexual persons. They make no such distinction. They say that
any legal recognition of a homosexual union is contrary to the moral law.
Second, marriage is a divinely instituted reality that pre-exists the Church. When Jesus came along marriage was elevated to the level of sacrament. But it exists apart from the Church. People of any religion and no religion can validly get married.
Third, marriage is foundational to civil society. Therefore, the state has a vested interest in protecting it. The state cannot simply “bow out” of recognizing marriage anymore than it can bow out of recognizing gravity. I understand the impulse to have the government get out of our business. But marriage is not just a private thing. The state has a duty to recognize and defend it.
The idea to have the state “get out of the marriage business” is short-sighted and would have profoundly harmful ramifications. This is really the “foot in the door” technique to the normalization of same sex relationships. If the State gets out of it (whether it gets out completely or resorts to a secular “civil union” thing), then they have succeeded in placing genuine marriage and same sex unions on an even playing field. That’s their primary goal.