1) Gay marriage is incompatible with Christianity (and for that matter, Islam & Judaism). If someone asks you why you oppose gay marriage, the only thing you really have to say to explain it is, "I'm a Christian."
God doesn't condemn anyone for who he is; so if you're attracted to the same sex, that absolutely, unconditionally doesn't make you bad, evil or "un-Christian." On the other hand, let me note that I do consider hating, tormenting, or bullying people because of their sexual orientation to be distinctively "un-Christian" behavior. As Billy Graham has said, “God will not judge a Christian guilty for his or her involuntary feelings.” However, God has drawn a clear line in the sand when it comes to homosexual acts. If you're gay, you're not allowed to act on it. If that seems harsh or unfair to you, well, sorry, but you'll have to take it up with God. It's His rule.
"Thou shalt not lie with mankind, as with womankind: it is an abomination." -- Leviticus 18:22
The people of Sodom and Gomorrah could tell you how serious God is about that -- if there were any of them left. So as a Christian, you can no more condone gay marriage than you could give the thumbs up to prostitution or wife swapping. That means if you're a Christian standing in favor of gay marriage, then you're a Christian who's standing directly in opposition to the God whom you claim to worship.
2) Gay marriage will end up infringing on religious freedom. The moment gay marriage becomes the law of the land, all sorts of First Amendment freedoms involving the free exercise of people's religion will likely be infringed upon as a consequence. No pastor should be forced to marry a gay couple. No wedding photographer, cake maker, caterer, or wedding planner should be forced to be involved in these weddings. No church or any other location should be forced to be the site of a gay wedding. Children will be taught in schools that gay marriage is normal, legal, and moral -- and it directly contradicts the teachings of Christianity, Judaism, and Islam. To create this special privilege for gay Americans would mean impinging on the First Amendment rights of more than 200 million Americans.
3) Civil unions could confer every "right" that marriage does. Gay marriage is not about "rights;" it's about special privileges. After all, every right conferred upon a couple via marriage could be just as easily conferred on a gay couple via a civil union, which is a compromise that could probably be had if gay activists wanted it. In fact, the biggest objection conservatives have had to civil unions is that there's a fear they'd be used as a stepping stone to gay marriage. So, let's be clear: there are actually no "rights" whatsoever at stake in the push for gay marriage.
Instead, what gay activists are looking for is a special privilege of the worst sort because it's as much about dragging everyone else down as it is about raising everyone up. To accommodate gay marriage, the whole meaning of marriage has to be warped and twisted. The religious content has to be taken out and marriage has to become just one more reason to file paperwork with the government.
So, this isn't really so much about marriage per se as it is an attempt to force society and religion to accept gay unions as every bit as normal and healthy as straight relationships, which will never truly happen. You might be able to intimidate some people into silence with political correctness, but the truth is still there and people know it, even if they don't want to be screamed at and accused of being bigots for pointing out the obvious.
4) Gay marriage may be where it starts, but it wouldn't be where it ends. Once the definition of marriage is arbitrarily transformed to make gay activists happy, there's no chance it's going to stop there. For example, you could make a much better case for polygamy than you can for gay marriage. It has a much more robust historical tradition, it's more consistent with religious values, it produces children -- there simply is no compelling, logical reason why gay marriage should become the law of the land without also granting polygamy the same legal status.
Furthermore, once that door is opened, where does it stop? How about brother and sister? Marrying the dead sound any better? How does man and dog strike you? Adults marrying children? How does marrying a tree or a clay urn hit you?
People get outraged by this sort of comparisons, but this isn't just speculation; these are unions that have occurred in other countries. So, if it has happened somewhere else, you can be sure some wacko will want to do it here, some lawyer will decide marrying your sister is a civil right, and some liberal judge will agree with him. Next thing you know, anyone who opposes it is accused of being George Wallace and trying to stop the progress of civil rights. Sound farfetched? Well, isn't that exactly what happened with gay marriage?
5) Marriage already has enough problems as it is without gay marriage. One of the weirdest arguments in favor of gay marriage goes like so: Marriage is already on the rocks. Look at all the people cheating, look at all the divorces; so why not gay marriage, too?
This is like arguing that someone has already accidentally eaten some rat poison; so why not give him some cyanide to go along with it? When someone's sick, you don't make him sicker, you heal him. If marriage has been tarnished in our society -- and it has -- we should be looking for ways to strengthen marriage, not weaken it.
Sure, if gay marriage were to become legal tomorrow, you wouldn't have legions of people who are already married running off to get divorced. But, gay marriage would further degrade the religious element of marriage, cut down on the "sacredness" of it, and make it less of a special event. That would cause people to put less value on marriage, make them less likely to get married in the first place, and make them more likely to get divorced. This leads to more children being born out of wedlock and kids from single parents are more likely to commit suicide, take drugs, go to jail, drop out of high school, etc., etc. in every category that matters than kids from two parent families. You can already see this starting to play out in Sweden, Norway, and Denmark and you'll see it happen here, too, if gay marriage becomes the law of the land.