This is a straw-man argument.
Let's turn this around.
You're a substitute teacher. You've been given an assignment to present abortion as a normal way of family-planning (or homosexuality as a natural alternative to normal heterosexual relationships).
Do you:
1) Follow the lesson plan without question?
2) Follow the lesson plan, but explain the truth (about abortion or homosexuality)?
3) Quit.
They don't talk about sex in kindergarten or first grade.
If I have a real problem with it, I would bring it up with the administration. I would also talk to the parents to let them know what is going on. However, I would not talk about it in the classroom.