My title is from a Huffington Post article in which the author argues that gays should never again back a presidential candidate, like Obama, who doesn't support gay marriage. While you can debate the effectiveness of such a strategy, it looks like it's already happening. Exit polls from November's election show a startling trend: Gays have deserted the Democratic party at rates far higher than any other group.
The chart below, from pollster.com, shows shifts in presidential voting patterns compared to 2004. Groups below the horizontal line voted more Republican in 2008 than in 2004. While Obama did amazingly well with almost every demographic, there are three groups where he lost votes compared to Kerry: "Small town" (-6%), "Decide last three days" (-8%), and "Gay" (-11%).
(Click on the chart for a version you can actually read)
Although I don't have the data to prove it, my guess is that this is a result of Democratic-leaning gays not voting, rather than gays as a group becoming more Republican. If so, this likely occurred more in safe states like California than in swing states like Wisconsin.
This trend will probably continue, and perhaps even accelerate, until the Democratic Party finally elects a presidential candidate who openly supports gay marriage. While it's unlikely to affect the top of the ticket, it could have an impact in close races further down the ballot.
In the past Democrats have almost always paid a price for supporting gay rights. In the future they will increasingly pay that price for not doing so.