Do you believe the Bible is infallible?
Do you believe your interpretation of the Bible is infallible?
Do you believe your actions, which you base on your interpretation of the Bible, are infallible?
Be careful how you answer, because if you answer "no" to any of them, then your argument is reduced down to just your personal opinion. If you answer "yes" to all of them, then you demonstrate that you believe you are infallible.
There are many societies that allow regulation of other's bodies. Socialistic and communistic regulations on Body Mass Index as one small example. My point is that feelings don't make "right". Societies frequently do affect the bodies of others.
If society decides collectively that polygamy and incest are wrong, for the society, then they decide such. There was a time when it was illegal in the U.S. to marry someone outside of your race. That changed. Today, it's entirely legal and almost universally accepted. Laws change. Times change. Get used to it.
Also you should probably stop saying "scientifically proven" since nothing is ever proven in science. Science deals with probability not absolutes. The more people try to falsify a testable claim, the more likely it becomes that the testable claim is true. It is never "proven" to be true, the probability that it is true just reaches a threshold where it is unlikely to be false.
It would be nice if people like yourself actually knew what science is.
I was discovering that life just simply isn't fair and bask in the unsung glory of knowing that each obstacle overcome along the way only adds to the satisfaction in the end. Nothing great, after all, was ever accomplished by anyone sulking in his or her misery.
1) Sin is subjective.
2) Define natural. Based on the definitions that I have seen, it certainly is.
"Never fear. Him is here" - Captain Chaos (Dom DeLuise), Cannonball Run
Mace Windu: Then our worst fears have been realized. We must move quickly if the Jedi Order is to survive.