Why is fructose not glucose the main energy source of sperm?
I have been searching this for a while and I got something of an answer, which I am posting below.  

There are primarily two advantages of using the polyol pathway and aldose reductase to produce fructose from glucose via sorbitol:-  

1) Quoting the book here, 


  Seminal fluid contains up to 11 mmol/L (200 mg/dl) of free fructose.
  It is the major energy source for the sperm cells in their
  all-important race for the ovum. The advantage of fructose over
  glucose may be that many bacteria, which compete with the sperm cells
  for the available nutrient, prefer glucose to other energy sources.


Thus, to avoid other competitive bacteria and cells from utilising the energy source of sperm, fructose is used as the energy source since the bacteria prefer glucose.  

2)Another reason is that the metabolism of sorbitol into fructose is energetically efficient way (no ATP required) as compared to glycolytic conversion of glucose into fructose(G$\rightarrow$G6P$\rightarrow$F6P$\rightarrow$F). Tapping the polyol pathway is energetically preferred to metabolizing fructose produced during glycolysis.This is only to show the possible evolution of the polyol pathway once the advantage of using fructose as the energy source was established. It does not explain why the fructose was advantageuos but just that if it was advantageous, the semen-producers would develop the currently existing machinery to produce fructose as the exclusive source of energy.   

Hence, sperms (seminal secretion) contains fructose as the primary energy source and not glucose.