“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.
However, these Christians and Jews would know that just because they were on the island together doesn't mean they are all married.