Here are some technical stuff(if you could call it that). Why women went it more when it's that time. If you have ever seen a female dog when it is on the rag, you will notice that it will hump every male dog in town 10 times over, but it has no interest at all at other times. This is caused by hormones that make the animal very aroused. Forgive the analogy, but women go through the same chemical changes to make them more aroused so they will have sex more often during thier fertile period. This also means that there is a high risk of pregnacy around the time of a women's period.
I would think it would be a man's duty to have sex with his wife during that time because that's when she wants it the most.
Note to the ladies:Men are that arousable 24/7, so you have to give us some credit for self control

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Man who go through turnstile sidesways is going to Bangkok.