I don't plan to ever have children.
Though, for starters, I wouldn't attach avoidant names to things... seriously, 'penis' and 'vagina' are correct anatomical terms; prancing about them is plain silliness.
"Tab A into slot B" would be the quickest, easiest analogy to use for the mechanical side of sex (porn would be a great graphical demonstration, though you can get in serious trouble for showing them that if my impression of the law isn't wrong). But I'd be more interested in explaining the ramifications, like pregnancy, STDs, and how to avoid them. The emotional aspects as well; what feelings it incites, and certainly how to recognize the difference between liking/loving a person and just lusting for them.
I'd be sure to add, however, that sex (and enjoying it) is completely natural. That there is absolutely nothing wrong with it, no matter what anyone else thinks. But, at the same time, that rushing into it before they're ready could be something they'd regret for the rest of their lives.
And to close... I'd tell them that I won't stop them chasing what they want. I'll give my honest opinion and any advice or help if they ask for it, but that it's not my place to rule their lives.
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I think it's incredibly sad that no small number of parents just don't want to teach their children important things like this...
Edit: @ the poster above, that image made my day. ♥