You are incorrect. Marriage has to be an institutional part of the law because it is the institution which promotes the formation of the next generation.
Society rewards the institution not the individual. Homosexual behavior has zero chance of producing or raising a next generation in a mother/father environment.
Homsoexual behavior is ONLY about recreational sex, nothing else. The law has never considered or involved itself in "love."
Since the government took over the hodgepodge of do it yourself marriage recording, the objective has always been to protect the next generation. Religious marriage is not relevant to the law. Only that the marriage was legally recorded. Religion in marriage is as irrelevant as love in the eyes of the law.
Unfortunatly you have fallen for the homo-propaganda talking point that "religion" is the reason to oppose civil unions or homosexual marriage, it is not. There have very good and very sound non-religious legal reasons to oppose homosexual marriage in any derivation.
I will summarize your points if I may, and please correct me if I misstate them. Basically, your argument is that marriage, as defined as one man marrying one woman, is protected in order to promote propagation. Deriving from this desire on the part of the state would be the civil benefits that are conferred upon married couples.
My response is that I believe this to be outdated and based upon antiquated ideas of human sexuality. I cannot fathom being sexually involved with a man, and I believe that this holds true for most, (if not all), straight men. The benefits conferred by the state in no way influenced me to seek out, fall in love with, and marry a woman. If your suggestion is that marriage benefits are the only thing keeping men straight, then I think you are off the mark a bit. Heterosexual men and women will continue to be together regardless of whether or not marriage exists. Now I agree with you that the institution of heterosexual marriage is the ideal environment in which to beget and raise children, and that the civil benefits of marriage promote this behavior. However, in what way would the existence of homosexual civil unions diminish this trend among the populace?
Couples get together because of the biology of sexual attraction, whether or not they are "wired correctly" (and yes, I do believe that homosexuals are somehow "miswired"). The state does have a valid interest in promoting a long-term, stable relationship. Children are certainly at the forefront of the benefits to the state. But they aren't the only one. If so, we could justify outlawing all new marriages where one of the partners were unable to contribute to the propagation of the species. There are other benefits to society begot by long term stable relationships as well.
And as for your statement that homosexual relationships are only about sex, I would respectfully suggest that you are woefully off the mark. One of my family members mentioned earlier was in a committed, loving, relationship when their partner was killed in an automobile accident. They had a house (and mortgage) together, two joint car loans, and a life full of love and memories. The partners family was against their lifestyle and didn't honor the relationship at all. As a result, not only was one life lost, another life was ruined financially and suffered much more emotionally than was necessary. I think that it is more than just about sex. And civil unions would discourage the casual sex in any case.
That may well be, but it misses the point.
The point is that it is simply not possible for our government, as currently constructed, to limit the definition of marriage to a man and a woman. Folks could pass marriage protection amendments to every constitution they could find, and the courts will still rule restrictions unconstitutional. The will of the people is already clear in this matter, but the will of the people don't mean a thing.
Since it is not possible to make the government and courts do what we want them to do, it is necessary to limit the amount of damage they can do. In the past 70 years, government interference in the laws of marriage and divorce has eroded the institution in every way. In our grandparent's day, marriage was presumed to be a lifelong commitment, and people only got divorced under the most serious circumstances. Today, people go to a wedding and openly speculate on how long it will last. The state has decided that people can dissolve their marriages at the drop of a hat, so it is now assumed by society that this is the only standard. The government standard has supplanted the traditional standard.
Government involvement in marriage was a mistake. Fighting battles about who can and cannot be married will only compound that mistake. It is time to get the state as as far away from possible from the things we treasure, because the state can only destroy them.
If you had a neighbor who routinely let her children play on a busy highway, would you let that neighbor care for your children? The state is ten times worse than the irresponsible neighbor, because the state will destroy what you love as a matter of course.