and "Half of the marriages in America end in divorce. What's letting gays marry going to do?" and
"Do two gays down the street getting married affect how you live? No."
I sometimes have trouble debating with liberals on this subject as well.
As others have said before, I think that the gay marriage supporters should be required to make the case for gay marriage. We shouldn't have to defend an institution that has been between one man and one woman for thousands of years. But since there is now a movement for gay marriage, we must try, I suppose.
I think that with gay marriage come various other insidious things. Once the government announces a lenient position on gay marriage, they become lenient on other issues most Americans find morally reprehensible. These include polygamy and marrying animals. It's just like with abortion: legalizing abortion has allowed euthanasia, assisted suicide, and sometimes infanticide. Once you let them get their foot in the door, they're able to get their whole body in. They take their new "rights" to the extreme.
Liberals tend to have the upper hand in the argument over gay marriage because their persuasions produce emotions in those who are wavering on the issue. Some of their arguments: "Why can't people of a different sexual orientation than yourself reap the same benefits of marriage that you do?" and "Half of the marriages in America end in divorce. What's letting gays marry going to do?" and "Do two gays down the street getting married affect how you live? No."
Also - and I'm not exactly positive about this - Catholic/Christian priests will be required to marry gays. This obviously goes against their beliefs, the Bible deplores homosexuality. You can't ask them to do that.
I'm also not sure whether I'm for or against civil unions. I think I'm against them because you know at some point they're going to want the real deal (marriage).
I hope this helps you. ;-)