Bush got reelected on the anti gay marriage vote, so it's obvious he's bringing this up now to boost his overall rating, so his Republican friends won't get tossed out of Congress come November.
I disagree that marriage is a religious institution. It might be to some, but not necessarily so. Two people could get married by a Justice-of-the-Peace, with no religious input at all. What marriage is for sure is a legal institution in all cases. Therefore, what difference does it make what gender the two people are?
Personally I could care less if two men or two women want to get married. That is there business and the Government should keep their noses out of it!