This was going to be a much longer post, but somebody just posted a blog about gay marriage, so I'm just going to touch up on some of the ridiculous arguments that pop up in conversations with people who denounce gay marriage (not just Christians). Here are some common excuses:

1) Marriage is between a man and a woman.
-This is the point of inequality, but for some reason people think that it's a valid argument against gay marriage. The reason this issue exists is because the definition of a legal marriage is too narrow and exclusive. The defining of marriage between a man and woman is compeltely arbitrary, considering what a marriage is - a union of (at least) two consenting adults. The main benefit of this is benefits that aid one of the partners after another dies or get ill. Beyond that, at least in today's age, everything else attached to is an extra.

2) Marriage is a religious contract/ceremony, and is subject to religion.
-Marriage in this day can and is often secular in nature. You must get your marriage certificate from town hall, not the church. Having your service performed by clergy is purely optional. By this line of reasoning, atheists would also not be able to marry. And what religion is marriage subject to? How many more does that put aside? Isn't this a violation of church and state anyhow, if they were somehow intertwisned and marriage required religion?

3)Homosexuals can't have children, and marriage is in place to promote having kids, so gays shouldn't marry.
- This would also exclude any couples that had no intention of having kids. It would also exclude any couples incapable of having children to to problems with reproductive organs or those who have had to undergo surgery to have them removed. It would also exclude parents who wanted to adopt. But nobody's talking about stopping them.

4)Homosexuality is responsible for AIDS, if we let them marry we'll have an AIDS crisis.
-This might be the biggest bullshit of them all. Endorsing monogamous marriage (currently) would only help to curb the spread of homosexuals if anything, I can see no reason why it would increase the amount afflicted with AIDS. However, sicne most gays that want to be married are probably in monogmaous relationships already, it would likely have little bearing on the AIDS situation.
Also, AIDS is prevalent in the gay male community because it was introduced into it, and it's a self contained system. There are less bisexual males tahn females, within the gay community, they are largely exclusive to males. Thus, the spread will stay within the gay male community. Note, however, that the number of AIDS victims among lesbians is very small. This is an even quicker way of shutting down this argument, ask them if, based on this fact, they'd still allow lesbians to marry.

5)Homosexuality deprives children of a mother and a father.
-Well, duh. But so do single parent households. And those have existed for centuries without problem. This seems to be more of an argument against non-conventional families more than anything else. The same kind of idea could be transfered to divorce families, and families where parents have remarried, step-parents, etc.

6)Homosexuality is unnatural. Evolution would have eliminated it!
-If you get the second part, you can be thankful that at least you're not dealing with an evolution denier. However, studies have found that in families with lots of gay males, the females of the same family often produce more children. This would offer an explanation for the existence of males via evolution. Dr. Zachary Moore of Evolution 101 (http://www.drzach.net/podcast.htm) brings up more about this on his podcast.
As for unnatural, it's prevalent all across the animal kingdom. You shouldn't have to look far for examples of homo and bisexuality in nature.

7)Children who grow up under homosexual parents are more likely to turn out gay.
-Firstly, so what if it did? Secondly, the studies aren't there to back this claim. There appears to be no correlation between gay parents and gay children. Just look how many gay children come from heterosexual families....

8)It threatens the sanctity of marriage.
People are afraid of marriage slipping outside the context of the nuclear family. It's already way beyond that though.
What does another group getting married have to do with your marriage? If you really want to fight for the sanctity of marriage, try to decrease divorce rates!(I'd like to devote another post to this later - Christianity doesn't create the greatest recipe for a good, long lasting marriage. At least, from what I can see.)

8)They have civil unions, isn't that enough?
-If they were the same as marriages, they'd be called it. There are glaring differences, mostly in what kinds of benefits come with it. Civil unions don't carry the same weight when it comes to shared medical coverage, access to the partner's retirement funds if something should happen, life insurance, etc. They are not equals, if they were the issue would really be moot.

10) If two men marry, why not to animals? Or polygamy?
-It's a slippery slope. Animals don't have the ability to sign and uphold legal contracts. It's a silly argument.
As for polygamy, polygamy SHOULD be legal, and I may post on it later. As long as it's between consentual adults, of course.

11)Majority rules in America. And the majority doesn't want gays marrying!
-People who say this don't understand the concept of freedom this nation was supposed to have been built on. It's freedom and liberty FOR ALL, not just the majority. The freedoms and rights are guaranteed to every citizen, regardless of what the majority thinks about it. To deny people this equality is simply wrong. It is up to us in this country to see to it that the minority doesn't get trampled by the ignorant and hateful.
By this same line of logic, these peopel would have denied ithe acceptance of interracial marriage, of women's rights, would have supported Jim Crow laws and segregation. There really is no difference, it's all discriminatory practice endorsed by the majority. Being in the majority in no way makes you right. Just look at what people think of evolution compared to creationism. Numbers are in no way relative to the truth.

These are most of the ones I've heard used before, and it's quite apparent how easily they fall apart. Any new ones you run across shouldn't be too far a deviant from any of these, and even if they are, they should be quick to pull apart when you think about it based on libertarian ideas of universal freedom and protected rights.