i had a client who is a naturopath, she has two teenage kids, one boy one girl. i asked her how she handled the whole sex thing. she said she got out some medical books that show pictures of stds, showed them to the kids, grossed them out good and proper. always use a condom!
that might not apply if your son and his gf believe themselves to be in a monogamous relationship, but it might have some use somewhere herein...maybe a video of a woman giving birth. not a pretty, hippie video, mind you. a really medical one. ::shiver::