I'm not as versed in evolutionary psychology as I should be, but I think much of what we consider 'natural law' is actually instinctive behavior. I could be wrong, but I'm neither a philosopher nor a psychologist.
Oh, and thanks for being one fo the few reasonable posters to this thread.
You seem to be an intelligent and reasonable person. Why not? A pleasure as well.
If it is merely instinct, how do we put a ethical or moral value on the instinct if it is simply what we follow? Why should anyone receive praise or condemnation for following instinct?
Secondly, if two different instincts are present in situations, the stronger one should win. But sometimes the weaker one must prevail in order for us to function as a productive society, whether it be family, work, etc.
For example, if someone is drowning I have two reactions, one for self preservation and one for saving the life of the person drowning. Obviously, if I'm going to be of any good to the drowning person, I must put the stronger instinct aside and help. What dictates the weaker instinct prevailing?
When it comes to fidelity, I must keep my zipper closed; for the good of my family and, ultimately, for the stability of the larger society.
But like the U2 song goes "Every time she passes by, wild thoughts escape". If a man acted on every urge, he could populate a small village in no time. But, since I'm married, it wouldn't be prudent for me to populate a small village...other than with my wife. Of course, she would have objections to that!
So, I've got to keep that instinct in check most of the time.
I look forward to your thoughts.