It doesn't matter what the reason for marriage was historically. What matters is that America is a free country where people should be able to do what they want with their lives regardless of any of their inherent characteristics, including sexuality. If a straight couple want to get married because they love each other, they can. If two gay people want to get married because they each other, they should be able to. The building block of America is not a man and a woman getting married, the building block of America is freedom. It is wrong when the government restricts the freedoms of racial minorities, religious minorities, and ethnic minorities and it is wrong when it restricts the freedoms of sexual minorities.
On another note, even if 99.9% of Californians voted yes on Prop 8, it would still be unconstitutional. The United States is not a nation run by the tyranny of the majority. That is why the Constitution was written and why we have a Supreme Court to interpret it.
P.S. Antonin Scalia can suck my dick. I hope that guy dies soon.