Why do men grab themselves when women walk by?

Another question along the lines of "Why do bald men only eat vanilla ice cream?"The question supposes that:

• The questioner has observed for all men
  
• All men have responded to the stimulus in the same way
  
• The compliance reaction (if it exists) is defined in some way to confirm its presence or absence
  
• No other stimulus evokes the same response
  

None of these factors is proven or confirmed.

The question'e rationale is not confirmed.