How can one prove something is a sin or not? Would that not mean you would have to prove the religion is right?
I used to be a Christian and I know the answer to this one... but it turns out, it doesn't take any religion at all to know EXACTLY what sin is.
Romans 2:14-15
King James Version (KJV)
14For when the Gentiles, which have not the law, do by nature the things contained in the law, these, having not the law, are a law unto themselves:
15Which shew the work of the law written in their hearts, their conscience also bearing witness, and their thoughts the mean while accusing or else excusing one another.
James 4:17
King James Version (KJV)
17Therefore to him that knoweth to do good, and doeth it not, to him it is sin.
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But, as far as the subject of homosexuality, the Old and New Testaments go far beyond this
self-awareness of sin and proscribe capital punishment in the Old and a nebulous judgement in the New.
Romans 1
(26) Because of this, God gave them over to shameful lusts. Even their women exchanged natural relations for unnatural ones. (27) In the same way the men also abandoned natural relations with women and were inflamed with lust for one another. Men committed indecent acts with other men, and received in themselves the due penalty for their perversion.
A reasonable person can depend on this definition: "To him that knoweth to do good, and doeth it not, to him it is sin." Being respectful of others should cover it.
"Love covers a multitude of sin." (1 Peter 4:8)