Perfection
The Great Head.
Yeah, I'd estimate about 1 out of 10,000 erections is used procreationally. The rest are just for fun.
TIME FOR BONER MATH
Well, maybe in 10,000 are used procreationally, but one in 10,000 erections that are used (IE excluding random boners, morning wood etc. that are "unused") wouldn't be. Let's presume on average, a man between 14 and 75 would use a boner a day, that comes to a bit over 20 thousand boners, if 3 or more are used in an attempt at procreation, then my point stands tall. Considering that on average guys have more than two kids and attempts to produce them more often fail then not, I think we can presume that the 10,000 number is false.
Also, if we're talking about Viagra erections we're probably going to see more used for procreational purposes.