Marriage is a concept which is recognized internationally. You can get married in one country, go to another country and have it be recognized. You can travel to, e.g., Germany and if your spouse has to go to hospital, you have spousal rights.
I'm sick and tired of the factually bereft argument that marriage is a religious institution and shouldn't be recognized by government. It is not a religious institution, and it has existed in many forms (including same-sex) for thousands of years. Religions can recognize whatever they want. They *can't* own marriage.
Making a half-assed attempt like allowing revisionist *nonsense* like marriage being "between a man and a woman" and replacing it with AltMarriage (domestic partnership? something else?) and giving that the recognition in the US that marriage currently has ... is a naval-gazingly provincial notion that ignores the larger reality.
Replacing marriage with AltMarriage in the US means US citizens have AltMarriage, so when they travel to other countries they don't get spousal rights. This could hurt heterosexual couples (who, not being religious, get AltMarried because that has the government recognition) just like it hurts homosexual couples (who the marriage-replacement-fantasists would deny the right of marriage to).
If a gay couple married in New Hampshire travels to the Netherlands tomorrow, they are married, and that marriage is recognized there. If they travel to another country, it might not be today, but it might be 6 months from now. What is almost guaranteed is that if they are AltMarried it will not be recognized as equivalent to married in other countries.
(1) Religions don't own marriage. Why should they get to restrict the rights of others? The answer is simple. They should not. EVERYONE should have equal rights.
(2) Any argument on this topic, to be worthwhile, has to consider rights internationally, not just within the US.