Thurston and Lovey were too old to procreate... and by all accounts more Monogamous than any real couple with that much money.
Skipper was as gay as one could be in that era...
Gilligan was his 'prison bitch', whether he wanted to be or not
Ginger was a Closet Lesbian/Bi, only being with men to trade favors in Hollywood
leaving only viable baby producing couple to be "...the Professor and Mary Ann" and in any case
The Professor likely concocted a Spermicide out of the local plants.