That's not a "rational" reason. It is nothing more than using religion to justify a pre-existing prejudice.<quoted text>
The rational reason to deny homosexuals right to marriage in some peoples eyes is that they believe that the sanctity of marriage is between a man and a woman. It would be against what they believe is morally right. It would be against their religion. That is the rational reasoning behind some, if not most of those, who vote against gay marriage. And, they have every right to believe that way.
People used Christianity to justify slavery and the subjugation of women. They were wrong then and they're wrong to use their religion to justify discrimination against homosexuals now.