I believe as an American that's your right. What about a wedding cake for an atheist, a Hindu, a Muslim or a Jew? Would you bake any of those?
I believe homosexuals should be allowed to marry because then they can have the same legal rights as straight married couples and because I don't see how someone being homosexual and getting married infringes upon the rights of anybody else. I don't see how being a gay married couple is harmful to anybody else. Polygamy I've never really thought about, but I suppose if a man wanted to have eight wives or a woman wanted to have six husbands, I can see how that might create a situation where young men can't find wives, or social and domestic issues in regards to mate selection. As for siblings; children born from incestuous relationships are biologically more likely to be sick or disabled than to be fully healthy, and that, for me, is unethical. In regards to peadophilia and marriage; a child can't give consent, and so such marriage should be illegal.
I don't want to be the arbiter of morality anymore than you do. After all, you assert your personal religious views (which can't be verifiably tested or quantified in any tangible way) to be the single real morality that all the world should abide by! lol. I, on the other hand, recognize the right of the individual to live by the lifestyle and the moral code they choose, so long as it does not infringe on another human's right to do likewise, which is in fact the making os a self-regulating society where no person's human rights are infringed. I believe gay people should have the right to marry if they so wish, to have sex if they so wish, just like you should have the right to disagree with it.
Again, incestuous relationships are biologically harmful. Incetuous reltionships are also an extreme rarity, and some recent scientific studies show that humans are programmed to find mates outside familial ties, to expand the gene pool. Even if incet were made legal, the vast, vast majority of people wouldn't engage in it.