When he was a young lad, my husband was in the car with his dad on a long drive. They drove past a field in which a bull and a cow were getting it on. Then the uncomfortable silence in the car began. My husband sank lower and lower in his seat as his dad kept looking over, taking a breath, clearing his throat, and then not speaking. Finally, his dad said, “Do you know what those animals were doing?”
“Yes, Dad.”
“Good.”
And there ended the sex talk.
We’d like our boys to be somewhat more fully apprised of the facts about the birds and the bees (and the bulls). We’d also like never to have those long stretches of uncomfortable silences. We want them to feel like they can ask or say anything, and so, from a very early age we’ve just given everything a proper name and kept the doors of communication open. No question is ever rebuffed. I never used this advice when it came to sleep training, but in terms of sex, I firmly believe that you should “Begin as you mean to go on.”
A penis is a penis because when you are 13, do don’t want to call that part of your body a “pee-pee” anymore and you need a word that is and has always been comfortable. Erections are normal and natural, and are to be enjoyed in private. We always asked permission before washing their private bits in the bath, and they know that permission must always be asked for and granted.
From day 1, we have always referred to the boys’ future partners as him or her. I never want there to be an awkward coming out moment for any of them, or for them to ever feel like their love for a man would be anything to reveal to us. We’ve never assumed those partnerships would be marriages, necessarily, and Family comes in all shapes and sizes.
They know the difference between a tummy and a uterus. They know the difference between procreation and recreational sex.
I’ve even begun a discussion about the exploitative nature of most pornography with my eldest. It was theoretical, but I figure I’m laying some ground work. File it under media studies.
Two books on our bookshelves have helped when the boys have anatomical questions. Boys and Girls and Body Science by Meg Hickling is good for younger children who are curious about where babies come from. I find it a bit earnest, but it gets the job done by presenting all the information as good science. 
It’s Perfectly Normal is another classic, and I like it for the variety of body shapes it depicts. The illustrations are all cartoon-like, but frank. I think there is probably something more up to date out there, but I have not felt the need to go looking. (Suggestions welcome, though.)
Really, the books are only starting points for quick questions or long discussions. These days, we get a lot of questions about the definition of slang words. They ask, I answer. (When I can. Sometimes I need to look things up. Sometimes those things surprise me.)
