I wish I could agree with you, because who doesn't enjoy a bit of righteous indignation? But as much as I dislike religion, it's overly simplistic to demonize the opponents of gay marriage as universally hateful.
My parents used to oppose gay marriage, not merely because an ancient text told them to, but because they belong to a dying culture that tied theology into a general, pervasive distaste for homosexuality. This distaste might be compared to the old fashioned aversions to something as extreme as interracial marriage, or as minor as facial piercings. These were commonly shared values, perpetuated and re-enforced in the same way all tenets of culture are. Mainstream culture in the western world no longer shares these specific values and has rejected them, along with many others. However, significant numbers of people who weren't a part of this cultural shift often find themselves out of touch with society, and cling to their cultural traditions like a security blanket; religious or secular. They justify their tastes in whatever ways they deem sufficient, parroting off whichever arguments the like-minded use that seem to hold the most weight. But whether they dress up their grievances as religious (opposition to gay marriage) or political (distrust of immigrants) or social (hatred of cellphones); their opinions are almost always just matters of taste: "that's not how I was raised, and I don't like it." This problem can be exacerbated by religion, which often insulates people inside its various sub-cultures. This creates a social bulwark against changes occurring in broader secular society; in much the same way as geographical or intellectual isolation does.
You're absolutely right that gay marriage takes nothing away from straight people; but to many traditionally-minded Americans with quaint, old fashioned sensibilities, granting two people of the same sex the right to get married is an incomprehensible notion that mocks a sacred tradition. That's they way they see the issue. The idea that we would view it as a matter of civil rights seems ludicrous to them because they view it through an entirely different cultural lens.
It's a combination of many things: fear of change, wilful ignorance; accidental ignorance; superstitious fear; desire to reflect and reinforce the values of their sub-cultural peers. Regardless, it's a damn shame. But the good news is that tastes can change, and ignorance can be addressed. We've seen a profound cultural shift in the acceptance of homosexuality over the past couple of decades; a shift that has touched plenty of these quaint, old fashioned, conservative oddballs who still live in an age much of the world has left behind. People like my parents, who watched TV and saw gay people, and found out they worked with gay people; and while they still find the idea of homosexuality a bit distasteful, they now view gay relationships as legitimate and consider their lack of marriage rights unfair.
The truly hateful are a shameful minority, and it's unwise and blinkered to judge any and all opponents of gay marriage by their standards.
