Showing the age of the strip, they use hair tonic to lube their dicks with. No doubt the oils in that hair tonic left their pubes nice and shiny as well moisturizing their balls--oh these are ingenious young lads! Mark comes clean about secretly watching them masturbate and, with two throbbing naked boners jutting up like skyscrapers in the air, his frat masters have a way for Mark to redeem himself. Any guesses what happens next?
For those who may not know, hair tonic products were widely used in the 1950's and 1960's to style hair (and actually for decades before then as well) by men. They were often freshly-scented and the oils made hair glossier looking, which men (and women) liked. The oils also helped hold the hair in place, making it easier to style. Because it was greasy, men typically used it either naked after a shower or after getting fully dressed so that it wouldn't get on their clothing. In contrast to today's manscaping of pubic hairs, back then it made sense while just fresh out of the shower to dab a little extra in the palm of your hand and give your short curlies a hair treatment as well.