Humans have a natural drive to eat, drink, sleep and procreate.  These actions are in accord with a natural law for species to survive and procreate.  Thus activities in conformity with such a law are morally good.  Activities that work against that law are morally wrong.  As an example consider that to eat too much or too little and place life in jeopardy is morally wrong.

This theory has two major variations on it.  For the theists there is a deity that created all of nature and created the laws as well and so obedience to those laws and the supplement to those laws provided by the deity is the morally correct thing to do.   For atheists there is still the belief that humans have reasoning ability and with it the laws of nature are discernable.  For atheists who accept this approach to act in keeping with the laws of nature is the morally correct thing to do. 

What are the laws of nature that provide guidance for human actions?  These would include: the law of survival, the natural action for  living things to maintain themselves and to reproduce, etc..

It is a major problem for this theory to determine what exactly those laws are and how they apply to human circumstances.

The theory also utilizes the Principle of the DOUBLE EFFECT:

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Under the Natural Law Theory two people of the same sex interacting to produce orgasms will be morally good or bad depending on whether or not such actions are in accordance with natural laws or not.

Atheistic Natural Law Theory:

If there are species on earth in which members of the same sex physically interact to produce physical pleasure then homosexual couplings amongst humans would be morally good. The purpose of orgasms would be more than to produce offspring.

PROBLEM: the physical record may not be all that clear and open to interpretation. There is evidence of same sex couplings in species other than human. How many cases or species are needed to conclude that such behavior is natural among mammals and fulfilling a basic physical drive in a non-harmful manner to the species is what is debatable.

Theistic Natural Law Theory:

God made Nature. God made the Natural Laws. God made humans. God gave humans reason by which they are to learn of the natural laws. God also provides revelation concerning god's will and wishes. In the scriptures there are passages dealing with human matters and they are interpreted to have been given as a guide for the moral life. So in addition to the physical universe which is provided for the study of humans there is also the word of god.

There is a passage in the bible where Onan is condemned because he did not go into the tent of his dead brother's wife and have sex with her so as to produce more children. (see two accounts below) . At that time it was the custom in the tribe that when a man died his brother would be responsible for his wife and take her as another wife in order to continue the tribe. Onan went into the tent had sex with the dead brother's wife but pulled out of her and spilled his semen on the ground.

He was condemned for doing so.

A. Was Onan condemned for entering into sex for a purpose other than having children? If so then all sexual acts other than intercourse between a man and a woman who are married and preparing to have children would be immoral. These acts would include: Premarital sex, extra marital sex, masturbation, homosexuality, oral sex, anal sex, use of birth control.

B. Was Onan condemned for not being willing to father children by his dead brother's wife? If so, then sexual acts entered into for a purpose other than procreation would be morally acceptable.

There are many people who take each of these possible interpretations of the passage.

Genesis 38:6-9 -- The sin of Onan:

This passage describes how Tamar's first husband Er was killed by God because he was wicked. Under ancient Jewish tradition, Er's brother Onan was required to marry and engage in sexual intercourse with Tamar. Widows were not asked whether they wanted to remarry. In many cases, the woman would have experienced the sexual activity as a form of rape -- something required by tribal tradition which they had to endure. Similarly, nobody consulted the widow's brother-in-law about his wishes in the matter.

Their first son would be attributed to Er. Because any offspring would not be considered his child, Onan decided to use a common and relatively ineffective contraceptive technique to prevent conception. He employed "coitus interruptus". That is, he disengaged from Tamar just before he ejaculated, and "spilled his semen on the ground." (NIV) God was displeased at this action and killed Onan also -- presumably because he refused to follow Jewish tradition.

This passage was used until recent decades by some Christian groups who maintained that Onan's sin was actually masturbation. The term "Onanism" was coined as a synonym of masturbation. This interpretation is no longer in common use.

Onan was the middle of the three children of Judah, son of Jacob and father of the tribe which eventually produced both Kind David and Jesus. His older brother died without producing an heir. In those days, it was customary for the younger brother to take his deceased brother's wife and provide that brother with an offspring. So, Judah, Onan's father, ordered him to do such.

According to the account, Onan realized that his biological son, produced in this manner, would not be considered his own. If Onan provided his older dead brother with a son, that child would inherit both the seat of chief of the tribe as well as the oldest's portion of the estate. It meant that Onan would be inferior to his own biological child. It also meant that Onan would lose "financially."

The laws of inheritance in those days required that the older brother receive a double portion. This meant that if Onan provided his brother with an heir, Judah's holdings would be divided four ways, with two fourths (or one half) going to this child while Onan would only receive one fourth. However, if Onan retained his status as oldest surviving son, the inheritance would be divided three ways, with Onan receiving two of those thirds or about one and a half times more.

According to the scriptural account, Onan insured his failure by practicing the most ancient form of birth control known, premature withdrawal. For this, God struck him dead.

The account says that Tamar was the name of the wife and her dead husband committed some sin so grave that God killed him, although it doesn't specify the sin. Now, her husband's younger brother commits a sin, with her, and he is struck down by God. This man sent to her to provide her dead husband with an heir, has sexual relations with her. He pulls out before ejaculation, spills his seed on the ground and dies on the spot.

1. One of the difficulties for natural law theory is that people have interpreted nature differently? Should this be the case if as asserted by natural law theory, the moral law of human nature is knowable by natural human reason?

2.How do we determine the essential or morally praiseworthy traits of human nature? Traditional natural law theory has picked out very positive traits, such as "the desire to know the truth, to choose the good, and to develop as healthy mature human beings”. But some philosophers, such as Hobbes, have found human beings to be essentially selfish. It is questionable that behavior in accordance with human nature is morally right and behavior not in accord with human nature is morally wrong. For instance, if it turns out that human beings (at least the males) are naturally aggressive, should we infer that war and fighting are morally right?

3. Even if we have certain natural propensities, are we justified in claiming that those propensities or tendencies should be developed? On what grounds do we justify, for example, that we ought to choose the good?

4. For Aquinas, the reason why nature had the order it did was because God had put it there. Other thinkers, such as Aristotle, did not believe that this order was divinely inspired. Does this alleged natural moral order require that we believe that there is a God that has produced this natural moral order? Evolutionary theory has challenged much of the basis of thinking that there is a moral natural order, since on evolutionary theory species has developed they way they have out of survival needs.

5  It is doubtful that one can infer moral principles forbidding adultery, rape, homosexuality, and so forth, either from biological facts about human nature or from facts about the inherent nature of Homo sapiens. 

6. Critics of natural law theory say that it is doubtful, however, that the inherent nature of Homo sapiens establishes laws of behavior for human beings in the same way as it may establish laws of behavior for cats, lions, and polar bears. It is especially difficult because so much of human behavior is shaped by the environment, that is, by deliberate and non-deliberate conditioning, training, and education.

7.  Two philosophers (Aquinas and Aristotle) integral to the theory have different views about god’s role in nature, which confuses the issue, especially when trying to decipher if the theory relies on the existence of god.

8. The intrinsic nature of humans as it pertains to establishing laws of behavior may not be the same for animals, which presents difficulties within the theory.

9.. Human behavior may be solely reliant upon the environment that one is exposed to, which includes social classes, education and upbringing, this opposes the theory.

Those who hold that religion and morality are inseparable link God with the base of morality. God provides the basis for a universal morality. Without God , they hold, anything is possible. Without God as the basis for morality all that is left would be a nihilistic ethics.  History provides ample evidence to refute such claims as there are Buddhists and Taoists and Atheists and Humanists who live moral lives and avoid nihilism as well.

Do Our Values Come from God?  The Evidence Says No  by Victor J. Stenger at http://www.colorado.edu/philosophy/vstenger/Godless/Values.htm 

Philosophers have attempted to locate another source of morality that would fill the void between the absolute, universal morality based upon religion and belief in a deity as the source of moral authority and the nihilism of relativistic ethics and the post modern moral malaise.