Bible actually commands married couples to have sex regularly and often ................. Of course, you have to increase the size of the tribe for their own protection. They didn’t have to deal with a population of 6 Billion in those days. Unfortunately the greatest drive humans have is the sexual drive, its God’s/nature’s way of telling us to keep producing. If we lose that drive its time for another species to take over. Put 5 woman and one man alone on an island and I don’t think marriage would come into play.
If it were 6 non-Christians then I may agree. But otherwise you're simply saying that Christian principles depend upon the state of the culture.
Depends.
If they grew up alone on the island (meaning no teaching about marriage, et al) and had no other interaction with humans, there would, by default, be a polygamous 'marriage' with the man being committed to caring for and defending all the women and children, and the women being committed to the one man (and also helping to care for the other women).
But if one man and five women crash landed on an island, and he was married to one of the women, it would likely be quite different, particularly if they held out hope of being rescued at some point.