@maxdancona,
Perhaps I can try this way of explaining it....
A principle is sort of a method for deriving rules.
Thomas mentioned Kant's first formulation of the categorical imperative.
Cat.Imp.#1 wrote:Act only according to that maxim whereby you can at the same time will that it should become a universal law without contradiction.
He showed how this principle can be used to generate rules as needed for a situation (like the queuing problem). You brought up some cultural preferences, and Thomas explained how to then use the principle to create a new rule for the new situation. As the "conditions on the ground" change, the principle stays constant, but the rules it creates may differ.
Here is Kant's second formulation:
Cat.Imp.#2 wrote:Act in such a way that you treat humanity, whether in your own person or in the person of any other, never merely as a means to an end, but always at the same time as an end.
Let's now look at the situation of you (American engineer) being placed in Cambodia or India. In Cambodia about 30% of prostitutes are under 18, and about 40% in India. For all intents and purposes in Cambodia and India child prostitution is acceptable (not against social mores by my definition, not against social relativistic morality by your definition). Would it be wrong of you to use a child for sex in your new country? Of course not! Why? Because it violates the universal principle of using someone simply as a means to some end, this is using a person as an object. This is disregarding the interests of a child. It is even immoral by this principle for you to tolerate the behavior in others, you should stop if it is in your ability to do so. This is the moral justification for NGOs like SISHA to intervene in other cultures/societies/nations.