Ok the ground idea is very simple

Lets assume there is an Island with 10.000 Dating age Woman and 10.000 Dating age Men, for now no Births or Migration

Lets say every woman has 10 Dates in the time we watch them and 50% of this dates go to 2000 Men, so 2000 Men with 25 Dates and 8000 Men with 6,25 Dates.

How could woman notice that they Date very much the same group?

Edit: So people flee in Details, its an average town, of average size, 21 Century Anglo-European Culture, Main religions are Christianity and Atheism, other people live there to but they are not of "dating age", so we end in something like a 80.000 Inhabitants City

Just pick a Random Middle European town