The main issue Bush ran on in 2004 wasn’t gay marriage
In several key swing states, including the critical state of Ohio, gay marriage was on the ballot. This was deliberately pushed for by the Bush administration and campaign, who hoped to drive the base out to the polls to vote against it and for Bush.
In any case, while gay marriage specifically may not have been the top issue explicitly campaigned on by Bush, it was certainly a potent one during the 2004 campaign (when Bush endorsed a constitutional amendment forever banning it), and the dogwhistling about "traditional moral values" and such was certainly a top Bush campaign issue. I should know; I lived through this campaign and it was largely my political awakening.