My main problem with your societal-reasons is that those were and are based off of Christian definition of what marriage is. When the government decided that they could start taxing and regulating marriage (which had ALWAYS been a religious endeavor until the government took over.) they took their definition from what Christianity said about it. So this whole societal-reason is has a religious basis which shouldn't be defined by the government.
I personally don't think the government should have a say about marriage at all, because it is a "spiritual" connecting of two people.
You seem to have misunderstood me! I'm not only talking about societal reasons in Western Europe including the Medieval period (heavily based on the Christian concept of marriage), but also
including the pre-Christian period in Rome and Greece. It shows a lack of historical understanding to blame this on Christianity; Greece and (pre-Christian) Rome were quite clear on the matter as well. Rome had an extraordinarily complex legal system, and marriage was a significant part of that (in fact, our legal system today is based, more or less, off of the Roman one). Marriage in Rome was strictly between a single man and a single woman. I personally believe that it is Roman culture which has associated monogamy with Christianity (Christianity was based out of Rome for centuries)—Jewish tradition had nothing against polygamy and nothing in Christian scripture explicitly condemns polygamy, but Roman tradition, on the other hand, did condemn polygamy.
Homosexual marriage would have defeated the entire Greek and Roman purposes for marriage, which (as abundant evidence shows) was to produce and educate children in order to strengthen the state. In Rome, at least (I am less familiar with ancient Greek customs), marriage was very much a legal matter, though the state did not keep a record of marriages (leaving that to individual families)—but consanguinuity, for example, was defined by the Roman government, and not by any religion,
even for those marriages which were explicitly religious (
confarreatio, etc.)! Among the Celts and Germans, religion was largely a personal affair: for some religious, for some not.
After the fall of the Roman Empire, Roman laws (and definitions of marriage) fell into disuse, and the "barbarian" practice became standard—that of marriage being simply the agreement of two individuals to raise children together. Eventually, however,
to protect women who thought they were married to a man who later denied having married them, the Christian (Catholic) Church stepped in and required record-keeping and witnesses. Eventually, in the modern Era, national governments took over the work of record-keeping. However, modern definitions of marriage are no more similar to "Christian" ones than they are to Greek, Roman, or even ancient Chinese ones: marriage as between two people capable of producing children together was universal, and had little inherently to do with religion: the Catholic church got involved, largely, to protect women and reduce bastardhood (a bastard's life was particularly hard in a Feudal society), though trying to ensure exogamy was also included in the Church's efforts.
