Marriage is not a fad to be cut to shape according to social whim. The father of modern anthropology, Claude Levi-Strauss, called marriage “a social institution with a biological foundation”. Marriage throughout history is society’s effort to reinforce this biological reality: male, female, offspring. All our ceremonies and laws exist to buttress nature – helping bind a man to his mate for the sake of social stability and for the sake of the child they might create.
When ever I hear the offspring argument I immediately consider polygamy vs. monogamy.
While it's "biological" to mate with the opposite sex, create offspring and families...
...it's even
more "biological" to pair off and practice monogamy.
Simply creating offspring with marriage without monogamy is polygamy, and that has typically been shunned in society under normal circumstances. (when gender ratios are close to even)
Marriage is much more than a reflection of the natural inclination to create offspring -- you can do that with many wives -- it's more a societal recognition of a pair practicing monogamy. Marriage enforces it within the pair themselves and helps to dissuade others who have not paired off from interfering - kinda.
Not all heterosexual pairings result in offspring - yet we don't nullify the marriage when they don't - because procreation ISN'T it's primary function/purpose.
Instead, marriage serves as a social tool to assist us with our natural inclination towards monogamy.
Homosexual couples share this tendency towards monogamy, and society should allow them they use of the same tool.
Things are just easier in pairs of two, just ask the Navy Seals.
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After monogamy -- then comes families.
Once a pairing is solidified in marriage, it's been customary to rear children. No argument procreation is somewhat important to society.
A stable household/family unit is critical to raise children to adulthood - pass down knowledge/skills - provide support and security - and eventually go full circle and have your children look after you in your old age.
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Long story short, monogamy trumps procreation any day ending in 'y' when considering "natural" reasons for society to provide/promote/enforce marriage.
Had
our society been nullifying marriages when pairings failed to result in offspring (and I'm sure in human history this has been customary in some societies), then I would accept the offspring argument for restricting marriage to heterosexual couples.