Simple answer: The "sinful" condition of homosexuality can not be proven or disproven, one way or another.\n\nComplex Answer: Sin is a concept presented by religion, which is based in faith, not science. By using the word "proof" you are asking for scientific evidence to support a faith based concept, which is not only impossible, but unnecessary. If you have faith, you need no proof.\n\nHomosexuality as a sin (or not a sin) is something that can no more be proven than the "fact" that Jesus rose from the dead on the third day. This is not to say that this didn't happen, but what we need to understand is that this is something that we take on as faith.\n\nHebrews 11:1 Now faith is the substance of things hoped for, the evidence of things not seen.\n\nHomosexuality is labeled a sin in Leviticus 13 (somewhere), and only by this account does the Christian world understand it to be a sin. However, none of this can be tested and proven through scientific testing.