I can walk out of my front door and knock on the doors of four heterosexual married couples who either can't or won't have children...I guess they shouldn't be allowed to marry?
Either the government can regulate marriage or it can't.
Around and around we go with the same tired arguments. Traditional marriage, one man and one woman, doesn't make a distinction for cases that don't fit the pattern of producing children. That emphatically does not mean that the reason marriage exists (and is regulated by government) has nothing to do with children.
Wonderful point to bring up. Leads directly into the purpose of government regulation of the contract known as marriage.
Marriage as a social construct exists to provide for and protect the future of the society it exists within. That is, the society establishes an ideal environment for the raising of the next generation of citizens. It has been proven that a nuclear family of one man married to one woman is the best environment for producing and raising children. (I'm assuming you'll agree with this last point.) So society subsidizes and encourages marriages that will have the potential to result in children.
Now what about the couple that has no children or plans to have no children? My wife and I were married 14 years before we had our first child. We decided to have children somewhat later than most. I know of several other couples who married with the intent to never have kids (all of which have kids now by the way). The point is that the environment that these couples create still falls into the category of most favorable for children. Even if they have none of their own, they may adopt later. (again I know of several who have done this)
Couples who can't have children can always adopt. And in some cases (I know of a few) medical advances let them have children later.
A 'homosexual' couple will never be the best environment for children. In fact a 'homosexual' couple almost guarantees that the child will be molested at some point (either by one of the 'parents' or by one of their friends). A child should never be left in the care of a 'homosexual'