well, for Catholics and Orthodox and Lutherans, marriage is a sacrament.
It has to be considered more than a contract between woman and man but a union of woman, man WITH God.
A civil union can be between man & woman, man and man or man and tree, whatever, that's gubmint, but the sacrament of marriage is holy
And government does all it can to try and extend its province into marriage.
I frequently post about the purely biological advantages to heterosexual marriage for reproductive and child rearing purposes. These advantages impressively benefit the man and the woman, but especially their children.
However, people are always able and willing to mess up a good thing.
Because marriage is a superior means of producing better offspring, it creates a situation of reproductive discrimination.
That is, many, or even most people are not optimal to produce offspring, for all sorts of different reasons, though they still want to.
Other people are seemingly designed to have and raise children.
Thus, people who should not reproduce are often naturally handicapped to prevent them from reproducing, *or* interfering with those who should be reproducing. This may explain the reason for things like homosexuality, prostitution, and some of the sterility inducing medical conditions.
However, the worst corruption people have achieved to marriage is the dowry. By turning optimizing biology into a financial contract, the system is perverted. Old men should not marry young girls, ever.
Dowry was and is so vile that it convinced many that marriage was just a social contrivance of little importance, and that animalistic reproduction is all that matters.
Reproduction which can be directed by government, not biological advantage. So marriage as an institution was attacked. Social enforcement of marriage was forgotten, though some of the enforcement has been duplicated by the courts.
Children raised by a single parent are 60% more likely to become criminal offenders than are children raised by two heterosexual parents. This is because unlike children raised by two heterosexual parents, which puts the children on a “success” oriented path, the children of single parents are on a “survival” path.
Just as bad is the fact that people only have a finite supply of energy with which to make and raise children. If they engage in empty, reproduction-less sex with multiple partners, they exhaust themselves. That is, they have less energy devoted to their offspring, reducing their strength and health, and their quality of life.