Archbob I'm going to forget that you're American for a moment. Only because the way you think about your legal system can be very different to how others think about theirs.
Just because you can legally do something doesn't mean it's the right thing to do. But legislation is there to help society function. Now I live in Australia, and I'm proud to be an Australian. Recently Howard passed the Marriage Legislation Amendment Bill that amended the Marriage Act 1961 to formally include the definition of marriage as the union of a man and a woman to the exclusion of all others, voluntarily entered into for life. It also added formally that unions solemnised in foreign countries between a man and a man, or a woman and a woman must not be recognised as a marriage in Australia.
Now The Greens and the Democrats both opposed the legislation - or at least some of their members did. But the Liberals, National and Labour all agreed on it - that's every major party in Australia.
You don't so-often see the parties cooperate like this. Why do we need the legislation? Well here's an example. In Australia Bigamy is a crime. However if Bob goes to a foreign country and marries two women at the same time, and brings them back to Australia, unfortunately we have no way of determining which Marriage to recognize and which not to - so we're basically forced to recognize both of them. It technically does not go against our definition of Marriage - but it would be a crime had it happened in Australia.
However, if Bob goes overseas and gets legally "married" to Steve and they come back, we are legally obligated not to recognize said union as a Marriage, because it goes against our definition of Marriage.
Bush, Blair and Howard all identify as Christian. They're not perfect, but they do their best. All three of them are opposed to allowing homosexuals unions to be recognized as Marriage.
The question you need to ask yourself before supporting legislation changing the definition of marriage is what does homosexuality share in common with heterosexuality? Because they're not the same - they're on opposite sides of the coin. And they're not equal.
It doesn't take a genius to work out that the penis fits the vagina. It's reproductive purpose is achieved only in heterosexual relationships. Sodomy is not the same or equal to heterosexual sex, neither is fellatio or cunnilingus. Sodomy is in fact extremely unhealthy, whereas heterosexual sex is healthy.
Men and women who have heterosexual relations are the natural biological parents of children.