The only thing he did not specifically mention is that, while many people support gay marriage, very few of them vote on that basis. So there are three groups:

1) Gays desperate for legal recognition of their right to marry.
2) Liberals, moderates and independents, who have no unanimous agreement on gay marriage, but few of whom use it to pick candidates.
3) Social conservatives enraged at the thought of two Obama appointees providing a 5-4 majority SCOTUS majority declaring gay marriage constitutional.

The second group is irrelevant; this issue is a non-factor in their choice for president, and the third group probably outnumbers the first by a fair margin. Worse, they would want revenge on Obama and to "reverse the damage," while gays have little to lose if Obama is defeated AFTER their constitutional right to marry is recognized. The only way their right to marry could realistically be threatened then would be an amendment to the US Constitution, to which the president is irrelevant.