Inconsistency in opposition to civil unions

I understand why many Christians oppose "marriage" when it comes to gays and lesbians. As a word to describe that relationship, "marriage" is a poor one. "Marriage" is (at least in my mind) an institution that is established by God, not by the state, and should therefore be defined according to God's definition of it (which I would say He has done in His word, the Bible, declaring that "marriage" is a relationship between a man and a woman— not a man and multiple women, a woman and multiple men, an adult and a child, or two or more people of the same gender).

However, what I understand less is why many Christians single out homosexual relationships with regard to ANY civic recognition of the relationship present? Why do we oppose, say, "civil unions" for homosexuals who wish to express their commitment in the relationship, but we oppose ONLY homosexuals in this way?

There are many civic (and by civic I mean, that which is governed by the civil authorities) implications to the recognition of a relationship that marriage (and currently, with a few exceptions, ONLY marriage) brings. There are rights: to receive family benefits on insurance; to have visitation access during hospital stays; to exercising certain legal and financial options; and regarding taxation and inheritance laws. There are also responsibilities: at base, to an implicit (if not explicit) commitment to fidelity and the longevity of the relationship; to exercise responsible care and decision-making of mutual interest (think of a person's role in determining the hospital care of his/her spouse). These are, of course, just to name a few in each case. In short, a relationship that is publicly, civilly recognized is substantially advantaged in our culture.

That the relationship is based (at least in part) on sinful behaviors and lifestyles doesn't negate the fact that there is nevertheless a real relationship present. Why should that "kind" of relationship alone face the opposition that it does from Christians? Have we determined that those relationships are illicit whose origin and essence is/was established (at least in part) by sinful acts? If so, they we are highly inconsistent in our exercise of this determination.

If we were consistent, we would likewise oppose the public, civic recognition of, for example, a marriage that is the result of an adulterous affair; marriages wherein the couple had sex before marriage; and other like relationships. So, for example, the probate courts would be required to nullify an inheritance for mistresses or illegitimate children; insurance companies would not be legally allowed to provide coverage for a spouse if he/she was the former adulterous partner of their current husband or wife (and the previous marriage ended in divorce because of the adultery); a man who fathers a child out of wedlock would not be allowed to visit that child should the child be in the hospital. And so forth.

My point is this: there are MANY relationships in our society that have as their basis some sinful origin. In many of these, that sin has never been admitted or confessed, no discipline was ever exercised nor the sinner restored, and no reconciliation ever sought or achieved between the sinner and the one sinned against. Are we ready to say that we should simply not recognize these relationships, real though they may be, because of the sinful origins?

If so, we have a lot of inconsistency to clean up.