Ethics

If you ever you question if something is moral, without invoking the supernatural, simply ask if it's sustainable.
If everyone who found themselves in a similar context and did that same action, would it be sustainable throughout time?
If yes then maybe it should be considered moral and accepted, if not, then it should be immoral and should not be done.
A broader 'Golden Rule' if you will.

So what is sustainable through time?
World Peace, resource conservation, colonizing other planets other solar systems other galaxies, at a pace that would sustain our existence.

Using resources in a sustainable way, treating people in a way could sustain relationships. Curing diseases.

This has its holes, and it's probably an argument that it is sustainable to murder a cheating husband. Although that becomes unsustainable if every man were to cheat. So then the task becomes cultivating circumstance to avoid the drive to cheat, and cultivating men that don't cheat, although murdering a cheating husband isn't very sustainable in most cases, as it removes a parent from the lives of his children, the financial support he provided, and causes a ripple effect of harm throughout the people in his life.

Tracing the ripple effect that would propagate from your action this becomes a simple metric to augment the golden rule, especially when it comes to 'doesn't bother me' circumstances.