Separation of church and state
I have an axe to grind here. Maybe you will grind it differently.
It seems to me the United States' original separation of church and state principle did not refer to legislating morality; rather, to previous governmental structures based on "divine right of kings" to rule. Kings got their mandate from the "official" church who declared the royalty in question legitimately mandated by the almighty to reign.
Today I hear people saying i"separation of church and state" assures nothing religious can intertwine with law or government,.
Discussing that with my son, the lad said times change--and Constitutional principles evolve with them. Therefore, the old "divine right of kings" no longer denotes "separation of church and state," and a modern secularist connotation is what it's all about. What the founders had in mind is not as important as an adaptive government.
Those who evangelized to abolish slavery did not do it on one-man-one-vote, or proto secular libertarianism. They wanted to create a model nation that made holy law from the Bible the law of the land and citizens the sanctioned rulers. Although people both defended slavery and opposed it with Biblical quotes, the minority anti-Constitutional evangelist fanatical Christians agitated to ban slavery because they considered it immoral. They asked others to declare it immoral and to demand that the government enforce _their_ morality.
By that standard today, why would it be a problem with separation of church and state to define marriage as a pact between a male and female? (I happen to support gay marriage, union, etc.) What seems to escape people yapping about church and state today is that the public morality is what the public majority says it is. Don't like it? Change it. Gay marriage becomes public morality as much as banning gay marriage.
Maybe this will raise some legal debate.