@Setanta,
Setanta, we have a pretty good real issue that illustrates the problem with absolute versus relative morality... namely circumcision.
Some cultures (including that of my ancestors) believed that cutting off a piece of the genitalia of each boy was a moral imperative. Failure to do so would lead to the harshest of social punishment (expulsion from the community).
Some cultures, such as 21st century America, believe that cutting off a part of the genitalia is optional for male infants (at the parents discretion).
And some believe that cutting any part of the genitalia of an infant is morally reprehensible.
This is a real strongly felt moral issue. Some people feel strongly on both sides of the issue.
My belief is that whether this is morally reprehensible or a moral imperative has everything to do with your societal context. There are different social contexts that feel strongly on both sides.
The problem with your solution, Setanta, is this.
All cultures... the ones that support genital mutilation and those that oppose it, went through the same period of evolution. They are both made by humans and enforced by human societies.
So how do you choose? If you are making an argument based on evolution or human nature you fail to make any distinction between one moral system or another.