I have not convinced myself of this notion but it has been bumping around in my head for a few days.
The first Republican overtures about the <sarcasm> metastisizing cancer bound to destroy America and turn us all into fags </sarcasm> that we know as gay marriage came up in mid August through September when Dean was starting his acendency. This was the time Rove was begging "please give us Dean" et al.
When Dean seemed to be the presumptive nominee getting this issue into the public eyes and ears would make an already "left" Dean even "leftier."
While I think this is a winner issue for us regardless in many ways, it just seems tailor made by the opposition for Dean. If Dean didn't make this an issue because of his popularity I am not sure that FMA and DOMA would have come up again so soon. Maybe Massachusetts and New Paltz and Multonomah would have come about on their own but I think this might be another great legacy of Dean.