I'm affected quite a bit if 0% of heterosexual couples marry versus 100%.
I know your argument for why we support heterosexual marriage (for the children, from what I can tell) but how exactly does that affect you?
Because the social statistics on children born to a married couple (note, "born to", not even "raised by") versus those born to single mothers are staggeringly different. The single biggest determinant of poverty among any group in our society is not race, gender, religion, or nation of origin, but the rate of children born to single mothers within that group. Those differences affect how productive our society is as a whole, and how much of that productivity is spent dealing with the various problems that children of unwed mothers creates.
It's in the interest of the rest of society to make that number as low as possible. And before someone goes there, it's not about getting people to marry *after* they have children, and it's not even about making sure that children are raised by two people who are married. It's about ensuring that when a child is born, we know who the biological father is and can legally require him to care for the child without requiring the mother to seek this via the courts or having paternity tests or dealing with a case where he's moved out of state, or any of a number of problems that can arise.
Why do you suppose so many states have adopted common law marriages? If a man and a woman live together as a couple for X years, the state assumes they're married. If the woman has a child, the common law husband is assumed to take parental responsibility. Note, that at no time in our history have we ever applied common law marriage to gay couples. The reason is quite obvious, right?
That's why it makes no sense to apply the legal status of marriage to a gay couple. It's not that they can't decide to marry on their own, but that the state has no vested interest in them doing so, and thus no reason to apply incentives or rewards to them for doing so. Let's not forget that the modern legal status is not "marriage". It's a set of benefits (mostly) granted to people who get married. Those benefits act as an incentive to get people to marry before they procreate, so as to avoid the problem I mentioned earlier.
And before someone say's "but people get married even without the incentives!", you're missing the point. They get "married", but they don't enter into the same legal contracts that we want them to enter into. If the marriages people would choose to enter into all on their own covered all the bases with regards to child responsibility, we wouldn't need a legal status by the same name. We need to get heterosexual couples to enter into a very specific type of marriage contract. We have no need for homosexual couples to do so. They're free to enter into any sort of marriage they want.