The evidence is unequivocal that the best environment in which to raise a child is a stable, loving heterosexual marriage, period. Now of course I would want to show nothing but support for parents who find themselves outside of that ideal arrangement through no fault of their own (like a widowed single mother). And that is not to say that some homosexual couples couldn't do a better job raising kids than some heterosexual couples.
But to consciously choose to conceive and raise a child in an environment that is known to be fundamentally flawed (i.e., no mommy or no daddy) is flat-out selfish. That many heterosexual couples have children for selfish reasons does not justify us compounding the problem by expanding the practice.
So while I don't find it practical or reasonable to explicitly prohibit such inferior parental arrangements from occurring, I see no reason why the government should legitimize through official sanction anything but the best possible choice. What the government sanctions it encourages.