"It's also a symbolic tradition which I would like to participate in, someday."
Symbolic tradition in which a male and female unite as one, not male and male.
"Marriage was an institution long before Christianity was a religion."
Again, Gen. 2:18-24 states that God formed "woman" from the ribs of Adam to be his mate. Unofficially the first marriage. It states God needed a suitable helper for Adam thus he created woman, not another man. So yes marriage was an institution loooong before christianity was a religion but it was God who formed it. It's the way God intended it to be.
"And there is no requirement in the United States that one be a Christian -- or a theist of any sort -- in order to obtain a marriage license."
True but it does require one to be male and female. This country was founded by godly men and personally I believe God lead this country to its freedom. Yes, you have freedom of religion in this country but the basis for all our laws and constitution is based on God and His laws.
But as it says in Matt 10:14 "Whoever does not receive you, nor heed your words, as you go out of that house or that city, shake the dust off your feet." 15 "Truly I say to you, it will be more tolerable for the land of Sodom and Gomorrah in the day of judgment than for that city."
In His Name, aaron