Henry VIII got married: you won't believe what happened next.
If you put 100 guys and 100 girls on an island, for the first few weeks most girls fuck the same 10/20 men. Then the other revolt, either by violence, or creating a bullshit religion where god told the to marry 1 one 1 so that all men have sex. Magic !
Theres also the issue or raising children when everyone was poor... better be a team
In real society most top guys (in any field) went through a phase where they had 2 to 5 concurrent gf
There's a reason are more selective in mating and and it would be beneficial for monogamy within marriage for a woman especially.
And if you really want to get tribal and old timey, wouldn't the other men just kill and take these top men's possessions on this island, and wouldn't the women need to come up with another option, if they did flock to the top 20?
That said if you want to go further, people on the forums most invested in the subject talk about a "dual mating strategy" for girls where they secretly fuck top guys while staying with lesser guys for ressources.. It's kind of a rabbit hole i warn you.