A question for christians regarding morality?

If one says that moral issues are rooted in a particular culture or social acceptance model, then what culture do we choose? Shall we choose one that adheres to Sharia law which condones honor killings? How about one that adheres to a 'survival of the fittest' form of law? Or one that that condones the killing of female infants? There are large and thriving cultures today that accept these moral norms. So it's not a question of whether cultural forces can produce a morality, history has shown that it certainly can. The question is which culture's morality will rule supreme. The Christian morality code is valid for the people who choose to live in the Christian culture.