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Where do you get the idea that natural = moral and unnatural = immoral? In the new testament natural is often a synonym for carnal, worldly, or sinful, and is contrasted with the spiritual and good.
The definition of "good" is "purposeful." All beings have a purpose by nature (inductive insofar that every being is doing something simply by existing; deductive insofar that the Creator made every being with a divine intent of it fulfilling its Final Cause), and so everything that exists is good. Evil or badness is therefore a lack of being, or a lack of purpose. Now, as the Creator made every being with purpose (otherwise He would not have made them), we can say that the Final Cause of all beings is the Creator Himself. Therefore, the nature of being is to incline towards the Creator, i.e., God. Humans are only capable of evil because we have free will and reason, which allow us to contradict our purpose of inclining towards God.
So to answer your question: all beings that are good are natural, and evil is unnatural because it is a contradiction of purpose. All of the above is from reason, not Christian divine revelation; though the two are mutually supportive. Contrast what I've said with 1 Corinthians: Paul certainly isn't a dualist. He doesn't think matter is evil and the spirit is good. It's only the nature of humanity that allows us to be carnal, but our true natural state is to be spiritual.
This is my favorite subject, and I'm going to refer you to the Wikipedia article on Thomism which I wrote if you have any further inquiries of the issue.
Hebrew authorities at least are almost universal in the view that the sin of Sodom was their lack of hospitality and cruel treatment of the poor and foreigners who entered their city. Various sexual sins in general are sometimes added lower down on the list, but it is their xenophobia that ranks at the top.
In the gospels when Christ Himself mentions Sodom his emphasis is clearly on their lack of hospitality, not on any sexual deviancy.
Sorry to defer you to an outside source, but this puts it more eloquently than I could:
There is nothing in Genesis 18 or 19 which could support his theory that a lack of hospitality was the crime that caused God to annihilate Sodom and Gomorrah. In Genesis 18 God said, "The outcry against Sodom and Gomorrah is so great, and their sin [singular] is so grave . . ." (v. 20). What was the sin which "cried out" for punishment?
Genesis 19 recounts the story of how Abraham's nephew, Lot, entertained two angels at his home in Sodom. Word got around that Lot had some visiting men in his home, and "the townsmen of Sodom, both young and old," gathered outside his home, clamoring for the two visitors to be turned over so that they could be homosexually raped: "Where are the men who came to your house tonight? Bring them out to us that we might have intimacies with them."
Notice what's going on here. The strangers had been shown hospitality by Lot and his family (vv. 1-3). The townsmen didn't cry out to Lot that they wanted to be "inhospitable" to the visitors, but that they wanted to have intercourse with them, which is something markedly different. Lot attempts to quell the mob by offering them his two virgin daughters, suspecting that because these men were homosexuals they would refuse. The entire account revolves around a single sin: homosexuality.
While it's true that later Old Testament prophets pointed out other sins the people of Sodom and Gomorrah were guilty of (Is. 1:9-20, 3:9, Ezek. 16:46-51, Jer. 23:14), it's clear that the primary sin, the sin which provoked God's wrath, was homosexuality.
If you examine the Old Testament passages in which God outlines the sins which would merit the death penalty under the Mosaic Law (Lev. 20:27, 24:10-23; Deut. 13:5-10, 21:18-21, 22:21-24), you'll see that homosexuality was condemned alongside such crimes as murder, idolatry, and blasphemy (Lev. 20:13). Search as you might, you won't find the Lord meting out the death penalty to persons guilty of inhospitality.
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