As many people have recently witnessed in San Francisco and Massachusetts, gay marriage has absolutely no effect on straight people who are married.
I don't think its possible for a straight person to comprehend what it's like to live in the closet, any more than it's possible for a white person to know what it's like to be called a racial slur, or a man to feel complete fear of being raped walking down a dark alley at night. It is completely outside their realm of experience, so they don't think it's all that bad.
These same straight people (at least one person who answered probably falls into this group) will then say we should just have civil unions, or some other second-class legislation that will give gay people a small fraction of the rights that married straight people get. Or better yet, let's take their rights away just to be on the safe side.
The problem is that people don't live very long, so they don't know that the world used to be radically different. As you mentioned, all left handed children were once scorned, and forced to learn how to write right-handed because left-handedness was considered the mark of Satan. About 30 years ago scientists came out and said that it was just genetics, people are born that way. Society evolved and we now accept left-handedness.
160 years ago slavery was legal, with arguments from the Bible as the primary support. We freed the slaves and the world didn't end. 50 years ago multi-racial marriages were illegal. "Activist judges" made them legal, and the predictions of Armageddon never came to pass. Around that same time people were predicting that divorce would unravel the very foundations of society, again very little happened after divorce was widely legalized (except more people who were living miserable lives were able to get divorced, perhaps.)
For every civil rights struggle that we’ve had in this country some people have predicted that if we do such-and-such God will reign down fire and brimstone. Each and every time we’ve evolved, and God has remained silent. I personally think it might be because God likes greater civil right for minorities, but call me a whacky liberal.