One common misconception in the same-sex marriage debate is the idea that the traditional legal definition of marriage is a violation of equal rights. Since this is an extremely emotionally charged accusation, it's difficult to get past it into a real discussion of the issue.
Here's the approach I usually take:
1. Nearly everyone who thinks the government ought to issue marriage licenses favors defining marriage in some way. That is, they favor excluding some combinations of people (polygamy, incest, etc.), not individuals, from the definition. Even judges. Even you!
2. You can't consistently argue that by excluding certain combinations of people, traditional marriage violates equal rights—unless you also argue to remove every single boundary from the definition of marriage and say anyone can marry anyone, in whatever combination of numbers they like.
3. If you're not willing to argue this, then you're for having a definition with boundaries, which puts you on equal footing with the traditional marriage supporters.
4. So the question is, which definition should we use? It's fine for you to argue that your definition of "two people who love each other" is better than my definition of "one man, one woman," or someone else's definition of "one man, multiple women," but we need to start off by understanding that we're arguing definitions, not rights.
It's not unconstitutional to adopt either my or your definition, as long as it's applied equally to every individual. Remember that the Constitution doesn't recognize rights for combinations of people; rights only belong to individuals. So one can't say that a man and five women have a right to get married; one can only say that each individual man or woman has the right to enter into marriage (no individual is excluded). This right is then acted upon according to the boundaries set by the state's definition of what marriage is—boundaries which are equally applied to every individual. You would like to equally apply the boundary of "two people who love each other" (excluding some other combinations), and I would like to apply the boundary of "one man, one woman" to each individual equally.
But I agree that the boundaries we place on marriage need to be relevant to the institution of marriage in order to be legitimate, so why don't we sit down and talk about the reasons why we each think the country should use our definition?
This definition-vs.-rights issue needs to be clarified. Otherwise, if you're arguing for the boundaries of traditional marriage, you'll enter the argument having already been unfairly declared an unconstitutional bigot before any of your reasons are explained (despite the fact that your opponent also favors certain boundaries), and anyone would be unlikely to listen to the reasons why you're an unconstitutional bigot. We have to get past this first barrier if we want to be given the chance to make our case.