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My response is that I believe this to be outdated and based upon antiquated ideas of human sexuality.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.
Homosexual behavior has zero chance of producing or raising a next generation in a mother/father environment.
I disagree. Stable families and households all contribute to the raising of the next generation whether the children live in those households or not.
On the issue of marriage and procreation, I agree with SilentServiceCPO's post.
"The benefits conferred by the state in no way influenced me to seek out, fall in love with, and marry a woman. "
I dont make money merely to be a taxpayer, but my natural instincts of ambition aligns with society's needs well enough. I do however, calculate the short-term cap gains versus long-term cap gains when figuring out when to sell... Same here. Society benefits when people behave in certain ways, and so it has an interest in channeling natural instinct along certain paths.
"If so, we could justify outlawing all new marriages where one of the partners were unable to contribute to the propagation of the species."
Marriage is as much an obligation as a right, so why bother outlawing something that is practically impossible to verify? What's the point? It is like passing a law to tell denture wearers not to brush their teeth.
Nevertheless, let it be noted that in some states you cant marry if you carry Siphylus.
Nor can you marry your cousin.
Do you think you should be able to marry an impotent cousin???
"Heterosexual men and women will continue to be together regardless of whether or not marriage exists. "
Yes, after we destroy the legal institution of marriage - surprise, surprise, sex&love etc. will still exist. The question is the health of the society overall in the configuration of relationships that result ... In "Dan Quayle was Right" ten years ago, the author who reviewed social studies on the impact of divorce and broken families on children concluded that Dan Quayle's statements about the superior nature of the commited two-parent family were correct. That two-parent family is NOT the short-term selfish interest of males, who'd rather 'play around' and be polygamous if not socialized to be one a one woman man...
The upshot is that the breakdown of marriage is NOT HARD to engineer once you have a breakdown in social mores. And once marriage breaks down, families, ie, children suffer.
"Boyfriends" are far far more likely to be abusers of children than biological fathers, for example, and the studies showed remarkable increases in juvenile crime, school underperformance, and mental problems for children in broken homes.
" I think that it is more than just about sex. And civil unions would discourage the casual sex in any case."
If it is 'none of Govts bizness' what gays do, why should we care one way or another?
OTOH, if gays - one group with above-average discretionary income - have more money because govt benefits of marriage now flow to these double-income-no-kids partnerships ... but we have an increase in child abuse thanks to breakdown in family, where we can no longer even say "husband" and "wife" or "father" and "mother" on parental consent forms for schoolkids... How is society better off?