1. No law or government regulation should limit rights based on gender
This is the crux of my position: one side wants two people, regardless of gender, to be able to share rights and responsibilities under a legal contract, and the other wants to make sure that rights are only afforded when specific gender conditions are met.
2. There is no good argument not to
Our government is founded on the principal that our principals can be amended to fit ever-developing morals and social structures. Anyone who claims that a moral code should stagnate instead of evolve with the people it serves is, at heart, a bigot. Woman's suffrage, civil rights, etc. all were, at one time, against tradition. Adjusting law to better serve and protect its citizens has not morally bankrupted this country.
b. Harms the "institution" of marriage
Until divorce is illegal, until marriage licenses require at least as much education and testing as drivers licenses, then the "institution" of marriage has worse threats to address. And divorce will always need to be legal, to protect the rights of the individuals within the marriage.
c. Government will force churches to gay marry
Equal rights for women has yet to force the catholic church to allow female priests. The relatively recent hubbub for altar girls was a decision made within the church from pressures of its parishioners, not any government. Similarly have various denominations chosen to allow female pastors following internal pressures and schisms.
Separation of church and state, remember? Historically, and presently, the government has been more detrimentally affected by religion than the other way around.
d. Gay parents aren't as good for children as straight parents
Many problems faced by the children of gay parents are likely due them living in and struggling against an intolerant society. I imagine children of mixed-race couples had a harder time than their peers as well.
Another thing is that sexual preference has nothing to do with ones ability as a parent; straight parents can have children they don't want by accident. Never happens with gay parents.
e. Why not civil union?
Separate but equal is never just. Either the government stops issuing marriage licenses at all (marriage being the responsibility of the various churches) and only issues civil unions to everyone, or they issue marriage licenses to everyone.
f. Slippery slope towards polygamy
And if we don't allow gay marriage, it's a slippery slope towards letting no one marry! Slippery slope arguments are useless cliche trash, this generation's domino theory (and look at all the countries that fell to communism!).
3. I want to be able to look at the progress made during my generation with pride, not shame
Anyone who refuses to see parallels with the civil rights movement is being willfully blind. No, it is not the same, but it is most certainly analogous.
And if we don't allow gay people to marry, they will just get dangerous, back-alley marriages.