You're leaving out the massive fights from states that did not want to recognize gay marriage, which could have easily gone the other way. The same process will repeat with marijuana prohibition and the states fighting it may succeed this time. What would have happened if gay marriage stayed as a patchwork of legal statuses? What will happen in the long run when marijuana is legal in many states but continues to send you to prison for decades in others? This is not a stable situation and the exact reason we need the federal government to do things instead of leaving it up to the states.