This is pure politics. If has nothing to do with whether or not you believe in gay marriage. It's blatant posturing by Republicans, who are increasingly desperate as the midterm elections approach. There's not a lot else to get people interested in voting on them, based on their record of the last five years.
But if you can appeal to the hatred, bigotry, or discrimination in some people, you might move them to the polls to vote against that big, bad gay married couple that one day might in down the street.
Its really no different than the strategy in 2002 and 2004, divide the country and push all public debate into a "us vs. them" mentality. In 2002 and 2004 it was easier for they could simply use 9/11 to do this. Now, with Republican leadership in foreign policy clearly out of touch, this tactic of division and seclusion has a greatly diminished chance of working for the Republicans in 2006. So they have returned to their favorite whipping boy in 2004, the gays, and added a new player to the mix. The "illegals". Expect the Republican theme in 2006 to be a constant harping of the need to be afraid of these two minority groups. Be afraid of the gays and the illegals.