If we consider the norm to be defined as what the majority of people do, can homosexuality be considered normal since it defines behavior that is clearly not what most people indulge in? And would that make homosexuality abnormal? And if it were abnormal, would it be wrong to validate gay marriage?

The statistical norm might be defined by what is true of the majority. But why on earth would we want to define the moral norm solely in terms of what the majority of people do? That would mean that, by definition, vegetarianism, atheism, and marriage between different races was wrong. It would mean that you were morally wrong if you were an abolitionist in the South or fought for equality for women in America in the early 20th century (I'm not sure when that became the majority position) or fight for equality for women in many countries today. For that matter, it would make it wrong to be a Jew or a man who goes to college or a firefighter.

Perhaps what you mean is that homosexuality could be considered biologically "non-normal" (it's not clear exactly what that might mean, since whatever we do is allowed by our biology). That may not be true, depending on what one means by biologically normal. But even if it were, it would not make it morally wrong, since lots of biologically "non-normal" behaviors may be moral, including, for instance, monogamy and vegetarianism.

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