I think we already messed with Mother Nature when we instituted monogamy, and condemned sex outside of it. (By the way, if you think the earth and everything on it are only 6000 years old, please stop reading here.)
What was the 'natural' state of a human female before agriculture? After she had attained the age necessary to have menses (a number which has dropped significantly for centuries now, generally believed to be attributable to better nutrition) she would have either been pregnant or breastfeeding most of the time. Don't even factor menopause into this, because prehistoric women (and men) didn't often live that long. Of course, I'm operating on the assumption that women did not have a lot of choice when it came to a man wanting to initiate sexual activity.
Normal menstrual periods would have been quite rare, and probably a sign of relative infertility. Look at all of the ancient religious teachings that describe a woman as being 'unclean' during this time, and you can see that there was some sort of preexisting cultural basis for avoiding such a woman.
Limiting a human female to having sexual relations with only one man insured that she would have menstrual flows before relations began, and whenever he was unavailable to impregnate her. Menstrual periods are a fairly recent occurrence, in the history of humans, in my opinion. This drug just restores the situation that existed prior to civilization's establishment of rules for female sexual activity.
In your world, life expectancy was less than 35 years.