There are a lot of factors that go into textbook selection, but the main one is Texas. No, really. Texas is the largest textbook market, so (unfortunately) we tend to dictate what can stay in textbooks, especially about health and science, and what has to go. Texas sets the standard in textbooks for the rest of the nation.
State boards of education and panels of teachers are the ones who make these decisions. Of course, there is a lot of money involved, too. At the textbook level, though, it is more likely (Texas) state and local politics that dictate what's in your health books.
In the case of stand-alone abstinence-only programs like Project Self-Respect and (ooh, I'm blanking out here) other of those week-long sort of courses in the schools, that's where that money for abstinence-only goes. Textbooks mostly have to do with the prevailing political attitude, but I think some assistance to the schools may be conditional on whether they teach abstinence before marriage in their health programs.
There's a good discussion and a little more background on this here: http://www.scarleteen.com/forum/Forum8/HTML/000840.html
"And when the answer that you want/Is in the question that you state/Come what may..." - Coheed and Cambria, "Blood Red Summer"