I know that a lot of people think that "equal marriage" or "freedom to marry" are just euphemisms for gay marriage. As I've studied more, and read more- that seems to be untrue. While it is true that the politics of marriage, and the fundamental nature of marriage, are being brought up in the context of same-sex marriage, the context and the import of the movement go far beyond gay rights.
Marriage has been changing for hundreds of years. It's become much less a machine for societal replication and stability of inheritance, and become much more a reflection of the importance we place on intimate human connection. Children born inside of marriage have the same inheritance rights as those born outside of it, thus nullifying its importance in inheritance. A man can no longer rape his wife, thus making the criteria for lawful intercourse both inside and outside of marriage the same: consent. These changes, made over hundreds of years, have transformed what was once a codification of male soveriegnty over a household into what it is today: a voluntary instutution based on consent and whatever the partners bring to it, from poor impulse control, to a deep and abiding regard for each other.
There is simply no longer any reason to deny any two people, who are not legally married to anybody else, who are old enough to decide for themselves, who are ridiculously optimistic enough to believe in the thing, access to the institution of marriage. When anyone can marry, marriage will finally reflect the romantic ideal that we pretend it does, instead of the archaic history that we're loathe to acknowledge.
I'd write this longer and thinkier, but, hey, I'm tired and hopped up on sudafed and I'm supposed to be reading for Contracts.