B737 NG "Sterile" Switch
 

I have noticed this switch on the P-5 panel next to the light switches on to FO side on several diffrent aicraft. What does it do?
I'm sure its not for pilot birth control!

Not such an unusual switch. It is probably a "sterile cockpit" light. These frequently illuminate a blue light overhead the cockpit door (externally to the flight deck) so that the cabin crew can determine whether "sterile cockpit" procedures are in force.

Sterile cockpit procedures are a method for minimising unecessary flight crew distractions during the critical phases of flight. They imply that the flight crew are observing a quiet/minimal distraction operation, with no extraneous chat. The external light indicates to the cabin crew that the flight crew are not to be disturbed with non-essential or non-emergency interuptions/distractions. Individual company procedures may require a "sterile cockpit" below an altitude of circa 15'000ft.

Either that or the switch ensures the death of whatever sperm (or ovaries) are left after all that increased cosmic radiation.

Correct. Your aircraft were built to US requirements where the Sterile Cockpit, Blue, light over the cockpit door is on below 10,000ft. FAA rules.
No chance that it will help with your extra curricular activities!!

If the Sterile Cockpit light is an FAA requirement, wouldn't we see it on all Boeing aircraft? Maybe it's just another one of those customer options, as I've seen it in some Airbus wiring manuals.