At the time of Prop 8, California had a Republican governor. It passed with 52.24% voting yes and 47.76% voting no, so it is not like it was a landslide in favor of it. Also, if you look at the results by county, you will see that the counties that voted for Prop 8 are not the counties that tend to vote Democratic.
One data point does not make a line. Do the constitutional amendments banning same-sex marriage generally happen as often in Democratic states as they do in Republican states? Are the bans approved in Democratic states with as wide of a margin as they are approved in Republican states?
Look at the states that allow same-sex marriage. Maine, Maryland, and Washington passed law by popular vote and all leaned towards Obama in the 2012 election. Delaware, Hawaii, Illinois, Minnesota, New Hampshire, New York, Rhode Island, and Vermont passed laws by state legislature and basically all of them appear to lean Democrat.
Also, to be clear, the US Supreme Court ruled that the Prop 8 proponents lacked standing in the Supreme Court, so its decision means that it “leaves the original district court ruling (overturning Proposition 8) as the final ruling in the case; it also means that the Supreme Court did not discuss the underlying substantive merits of the case, and the case has not resulted in a formal precedent for states other than California.”
Regarding marijuana, Washington also legalized and it appears to lean Democrat. There are also multiple states where marijuana is decriminalized and/or legal for medical reasons. It is interesting that you point to Colorado as an example of a Republican state when it voted for Obama in the past two elections and elected Democrats as governors, lieutenant governors, and US senators, and generally Democratic majorities in the state senate and state house.
Focusing on a different subject, access to clinics that provide abortions and other health services for women, this shows how multiple Republican-led states are trying to close clinics: http://video.msnbc.msn.com/rachel-maddow/52446472 This is not one Republican governor or one Republican state legislature, this is multiple states.