Let's review the social dynamic.
Four men and three women stranded without chance of rescue.
Two of the men, the Skipper and Gilligan bunk together in a loyal brotherhood of sea-goin' Stan and Ollie-like commitment. Later Gilligan would venture to the Island's hemp fields and escape reality.
The Professor pursued with due diligence the study of wire-less cocoa-nut radio... in it's infancy at the time, and was thus preoccupied.
Thurston Howell III was therefore the only male on the island with a shot at the girls. And Ginger, being used to money and privilege, along with Maryann, needing a father-figure, no doubt were giving Mrs. Howell some competition. What were we talking about?
Oh yeah, I guess Ginger- sweet as ginger-ale and gingerbread. No wait, Maryann- warm and caring. I don't know, too hard to choose.