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.
“Put 5 woman and one man alone on an island and I don’t think marriage would come into play.”
“Depends.”
Yes, if they were Christians from Ethiopia (where Roman law did not influence early Christian practices) they would have no problem with polygamy. If they were Jews who knew what the Bible said about polygamy they would have absolutely no hang-ups about it either.
Unless it is Slick and Hillary and a few other he and her can fight over.