The point that many make including myself is that the government can no more ban homosexual sex that it can ban murder. Regardless, we see laws against murder. These ;aws are not created because the laws will or can even hope prevent murder but rather because the laws clearly define what society declares necessary and valued conduct supporting moral order versus anarchy.
So, although we can not prevent murder, we can state it is not good and as well not see the government promote it as normal in public or on the public dime...
Codified laws and government may come and go -social order and the common law is the legitimate foundation premise that underlies what is both legitimate governance and legitimate legislation. The declaration of no value regarding homosexual sex was made long before government began as an agent of society incorporating it into written law.
What we see now as evidenced with Proposition 8 is a tyrannical government imposed morality/religion that claims it knows better what is morally right. Again, where does this moral authority come from -the holy book of homosexual sex?
So, given the choice, you would not outlaw sodomy?