There are plenty of things people do that I don't like. I don't like couples who sit on the same side of the booth at a restaurant. I don't like families who dress their children alike. I don't like the spelling "Aymie" for the name Amy. I could form a religion based solely on the belief that these things are wrong. If I tried to put something in place making these things unconstitutional, people would flip a shit because the idea of it is just ridiculous. My religious beliefs should not be able to dictate others' personal decisions to sit where they want, do what they want, etc. To me, this issue is exactly the same... but because it's Christians raising a stink and there is enough support, it might actually go somewhere. Peoples' religious beliefs should not have any legal clout over what others are or are not able to do. Period.
or, another simple reason:
We get three things. Life, Liberty, and the Pursuit of Happiness. If you think those are good founding principles, allow your gay citizens the ability to pursue happiness.
*edit to add: I guess the last line of my first bit is an absolute... sorry.