(Photo: Ian Robertson)
Human sperm cells need to be cooler than the normal internal temperature of the human body, so it's helpful that the testicles are housed externally in the scrotum, which is conveniently air-cooled.
Well, at least it can be--if you wear a kilt! Before humanity was cursed with pants and later with form-fitting underwear, men wore clothing that permitted their testicles to remain cool. E.J. Kompanie writes in the Scottish Medical Journal that men who wish to sire offspring should do likewise. They can do this by wearing kilts, which could increase the likelihood of conception:
Wearing a Scottish kilt in a traditional ('regimental') way may have clear health-related benefits. Kilt wearing likely produces an ideal physiological scrotal environment, which in turn helps maintain normal scrotal temperature, which is known to be beneficial for robust spermatogenesis and good sperm quality. […] Based on literature on scrotal temperature, spermatogenesis and fertility, the hypothesis that men who regularly wear a kilt during the years in which they wish to procreate will, as a group, have significantly better rates of sperm quality and higher fertility.
-via Seriously, Science?