I understand that we’re not required to accept the mores of another culture, but what argument is there that we should accept the ethical system of our own culture except as a practical matter where it is in our own best interest to do so?
If it is in your best interest, that is sufficiently good reason to follow the ethical system of your own culture.
A personal ethical system is nothing more than our own set of preferences, indistinguishable from our preferences in food or entertainment.
I already explained that there is a fundamental difference between a
personal behavioral system and an
interpersonal behavioral one. What you do to your own body is no one else’s business. If your dietary preference is to gorge on junk food, it is your decision. You may get some illnesses, but it was your choice.
On the other hand, when it comes to interpersonal relationships, then you need to consider the society you live in. If the society is predominantly Catholic, you had better follow - or
pretend to follow - the generally accepted moral customs. Otherwise you might get ostracized, or even get burned at the stakes - depending on the era you live in.
If you live in a predominantly permissive society (the atheistic Scandinavian countries come to mind), where you only need to worry about keeping the laws, your personal freedom is left intact. You are free to worship any god or not to worship any of them. You can be free to be sexually active and no one will care about it.
So now we have two societies based upon their generally accepted moral systems. It is obvious that the one, which imposes fewer limitations of your personal freedom is preferable to the one with more restrictions. (Don’t forget that restrictive society
COULD BE a Muslim one, with Sharia laws.) So the social norms
CAN be compared to one another and one can find an objective preference. The permissive society is clearly better for everyone (you included) than a restrictive one.
That is the difference between the social norms and the dietary preferences. And that difference in enormous.