"I'd imagine you're aware of the usual response to that, but I think it's important enough that it bears repeating.
Whether or not being gay is a preference/choice or biologically determined is is irrelevant to the arguments in favor of gay marriage."
I don't really see any contradiction between something being a preference/choice or biologically determined (I consider all preferences and choices biologically determined).
"Regardless of the cause of homosexuality, the government should not be in the business of regulating the behavior of consenting adults."
That is of course a perfectly respectable libertarian standpoint, but most people are not libertarians. On the contrary, they have no problems with the government setting up institutions to promote what they consider pro-social virtues and behavior. Such as marriage.
"If that's the case, it's odd that so many of the legal and financial rights/privileges granted by marriage do not directly relate to child reading and, indeed, apply regardless of whether or not the married couple has children, ever plans to have children, or even can have children. For example, my mom is now past child-bearing age. Does that mean she shouldn't be allowed to get (re)married?"
Child rearing is one of the aspects behind people's support for marriage, but far from the only one. Marriage plays many other important roles in regulating the interaction of the sexes.
"PS - As a side note, part of a well-functioning government's role is to protect minorities from tyranny of the majority [wikipedia.org]. So while you're right, a minority population can't count on the state's protection, it's not unreasonable to expect such protection in the (theoretical) 'ideal' state."
Of course, one can count having ones preferences catered to an essential right, but I consider that attitude more than a little narcissistic.