Laelia wrote:
Lots of same-sex marriages don't involve children born from that relationship. Straight people have kids before getting married, or divorce and remarry, or adopt, or get sperm/egg donations or surrogates. Almost all of those options are also available to gay couples. Even if you think the sole reason behind marriage is protection of children, there's still no legitimate place to draw a line between straight and gay couples.
All of that is inherently political.
Parents produce children through copulation and marriage is there to facilitate that. Do heterosexual couples adopt? Sure. But who can adopt, under what conditions, etc, all of that is political even if it's not necessarily controversial. It's a question of our values and preferences.
Do we allow polygamy? No, but some cultures do.
Do we allow single-parent adoption? No, but some cultures do.
Do we allow male primogeniture? No, but some cultures do.
Do we allow absolute power of life or death over their children? No, but some cultures do.
Do we allow child abandonment or infanticide? No, but some cultures do.
In a diverse society based (at least putatively) on tolerance, it is logical to minimize government involvement in purely human affairs to what is culturally universal. To what can be demonstrated as fundamental to the human character. If people want to form their own values and live their lives as they please then they are free to build a life beyond those basic parameters.
That men and women pair off and make babies is culturally universal. Marriage is something that exists in every culture, and the belief it should be engineered in some way - and to what extent it should reflect the values of a given culture, in ways that are beyond strict human nature - is a matter of cultural preference, backed up by the political system.
The "legitimate line" is between a man and a woman. Period.