Having said that, the issue you're dealing with is quite a bit more difficult: because your pro-gay-marriage friends can simply counter "OK, for the sake of peace we will drop the attempt to call it 'marriage' and simply call it something else. But you must extend to us the same legal rights as a married couple." Honestly that's a tougher nut to crack. Personally I think that it is wise to begin strictly with the legal issue, and then move to the moral/social issue.
The legal issue is this: the Constitution of our government in no way requires gay unions to be recognized as equal to marriage. Nor, however, does it require gay unions not to be so recognized; it is silent on this issue altogether. In particular, I do not believe the equal protection clause can be used here: after all, if a gay man and a gay woman want to marry each other, they are perfectly entitled to do so under the law. Now I know that sounds absurd; but it follows logically from my previous argument about the very definition of marriage.
The social issue is, of course, whether monogomous marriage should be given special status compared to other unions. That's where the toughest argument lies---especially with those that do not accept the moral authority of Scripture. I think a possible line of attack is to suggest that the state's interest in limiting marriage to a man and a woman is in the acknowledgement that it is the best environment in which to raise a child. Of course, many damaged marriages are clearly no place to raise a child; and there are plenty of "family" structures that do not involve marriage in which children are successfully raised. Nonetheless I believe that it is intellectually dishonest for anyone not to concede that the ideal parental structure is that of a father and mother in a stable marriage. For that reason, the state does indeed have an interest in differentiating between marriage and other unions.
And even though we as a society have greatly weakend the institution of marriage through divorce, abuse, adultery, and so forth, that does not excuse the government from weakening its resolve to protect and defend the institution; on the contrary, it should work harder to preserve, uphold, and restore it.
The slippery slope argument is another angle of attack: that is, you could argue that if gay unions are recognized, then polygamous unions, incestuous unions, and so forth must be recognized as well, if there is full mutual consent among all parties. You might ask your pro-gay "marriage" compatriots where they would draw the line? Would they accept a mother marrying her own son (or daughter)? Honestly I don't think this is a particularly strong argument in itself, but it's a useful arrow in the quiver.
One argument is that the government should get out of the marriage business altogether: that all unions should simply be enforced by contract and not by a pre-determined legal framework. This is certainly the most libertarian and/or egalatarian approach. I do not agree with this simply because I do believe the state has an interest to uphold the institution that best serves its most defenseless constituents (children). Frankly it's just as destructive an option for our government to take.
Anyway, please forgive my verbosity. I'm just trying to give you whatever I can think of in short order. I'm sure I've missed something.
YOU have a FR account?
I gotta know -- do you laugh, get angry, or just not react at all to being called "Hildebeest", "Hitlary", etc?