This one learned for himself about sex, (mostly from Biology classes, documentaries, and the occasional sex ed class), and I expect any potential spawnling of mine to do the same.
However, if it asks me directly, it will be directed to google, or if one is feeling lazy, tell him to read some biology books, or watch the discovery channel...
My parents only ever said this to me:
"Make sure you use protection, as you don't want to make a girl pregnant and ruin your future"
"Trust me, focus on your education rather than girls"
"Don't marry your first girlfriend, we regret it" (That one is a different issue in its enirety, and I made sure they knew that I knew that! >

)
EDIT: These exact phrases may be the real reason I am still a Virgin, and have NEVER been in a romantic relationship with a woman, (I'm perfectly satisfied with the mango tree).