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All morality is based upon some sort of guide - a rule or a plumb line by which it is measured. I'm assuming that the "common man morality" that you refer to is based on today's society or culture. Unfortunately, the idea of using society as a guide is flawed, as society (and those who influence it) are not consistent.

As others have mentioned, why is that a problem? Without that changing plumb line, we'd still be in a state where girls with pierced ears are immoral, interracial marriages are immoral, torture is moral, stoning people to death is moral, nudity is moral, masturbation is immoral, women showing the skin on their ankles is immoral, and on, and on, and on.

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The idea behind a spiritual morality is that it is based on an unchanging force or being.

Which is a fine idea, but is meaningless as a method of determining how humans ought to behave, because no-one can agree on what it is.

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Thus the conundrum that is vexing this conversation: if those who claim to be Christians adhere to the Bible as absolute, unchanging truth, there is no way they can accept homosexual marriage as a valid institution.

And they ought to still be running around stoning people, too, but they're not.