This has nothing in common with girls wanting to be on the football team, polygamous families, or bestiality fetishes. Those are distracting arguments.
Quite simply homosexual couples are seeking equal recognition under the law of their unions by having them called marriages, and therefore enjoying all the rights that follow; property rights, end of life decisions, tax filing, etcetera. I've heard no fact based argument against same sex marriages. Homosexual marriages don't "redefine" marriages for all. Marriage is a union, and life commitment between two people. The commitment of two homosexuals does not weaken the commitment of two heterosexuals. These marriages are independent of each other. Two lesbians, or two homosexual men who have made this commitment should be able to make end of life decisions, or pass property to each other without the extra-legal work needed under current law. As many smarter than myself have noted the social construct of marriage has evolved from its definition since medieval times from marriages outside of social caste, race, faith, and so on.
As to families and child rearing, there is no clear evidence that children raised by homosexual couples have significantly different outcomes than kids raised by heterosexual couples. Research does show that children do best in households where both biological parents are present in the same household for their complete growth and development. 50% of traditional marriages end in divorce, so even half of the preferred settings don't meet the standard. Comparing heterosexual households with one biological parent, and homosexual households with one biological parent, or adoptive households with zero biological parents (hetero and homosexual) there's really no difference when it comes to child rearing and development.
http://en.wikipedia.org/wiki/LGBT_parenting
There is no compelling argument that marriage equality for same sex marriage will in any way affect the liberties of opposite sex marriages. Those who oppose same sex marriage equality have done little more than argue the "social order" or "traditional norms." All that really means is they're uncomfortable with it, or that it doesn't fit their definition of normal. Why and who cares? It's not your marriage. It doesn't affect you in any way, shape, or form.
