I didn't know he was sterile...
I did know that he didn't impregnate anyone though, one obvious advantage of this is the fact that Solid Snake, Solidus Snake, and Liquid Snake could have been geneticaly altered to become the perfect soldiers.
Yeah, at some point in his life he became sterile

Then when he was 50 they decided it would be a good idea to clone him (thus why they age faster, they start off having 50 year old DNA...that's their explanation, anyway...I don't see how it could affect things that much since DNA itself can last thousands of years). And somehow they managed to give Liquid all the recessive genes, Solid all the dominant, and make Solidus a normal blend.