a) A woman's reproductive system is not affected by stress.
or
b) Rape is not stressful.
Think about that for a damned minute. Every species on the planet is designed to endure stress and prosper. If all it took was stress to prevent pregnancy then civilization would have died out a dozen times in recorded history alone. One of the greatest stresses there is physically is lack of food. Yet women in Ethiopia kept on managing to get pregnant.
This is reminiscent of the medieval test if someone was a witch.
Throw her in the river. If she survives, she must be a witch and must be punished.
If she drowns, she must NOT have been a witch.