OK let us start with the medical reasons why women who were menstruating were kept away from sexual contact and for 7 days after.
Many people who are not MD's or nurses cannot deal with this information because it is not taught to the public unless you go to med school or take courses in female diseases.
It is well known that any infections at conception destroy healthy coding in the babies. This is the molecular basis for not having diseased sex. This is also why women have two forms of protection against infections. But if one is completely destroyed from bad choices in life, the baby has no chance to develop without a new form of disease.
Here is the zika virus that is an extreme of this effect. If you have any viral infections at conception the baby will have deleterious mutations from the way that viruses infect and change DNA in cells. ANY viral infection that looks for a "host" cell in your reproductive cells, sperm, egg, or first cell so of the zygote, can cause genetic mutations. It is the nature of viruses to infect and to impose changes to the host cell's DNA. Never in all of science has even ONE beneficial mutation ever been found.
It is well know that if a woman has sex during menses she can easily contract a disease because the pH of the Blood at a pH of 7.4 vs the pH of the vagina of 2.7 to 4.0 cannot stand to have the pH raised for any length of time. It kills the bacteria flora in the vagina that stops infections. That bacteria produces hydrogen peroxide. This is why tampons that were not clean caused many women to die in the 1960's and 1970 from "toxic shock" in their bodies.
When the blood is contained by a tampon, the pH of the vagina is raised and so it must really be allowed to bleed naturally and flow out to do what was intended. Blocking the flow of this natural process is very bad.
Now if you raise the pH it will eventually destroy the flora. If a woman were to have sex and using tampons the odds are very high of contracting a bacterial infection. Sperm has a pH of 8.0 about 14,000 to 16,000 times more base than the flora and natural pH of a woman.
pH has a logarithmic scale base 10. That means each number is 10 times more or 10 times less acid from the prior number.
A pH of 7.4 is approximately 10,000 times more base than normal vagina pH.
Starting with the vagina pH of 3 average going to 4 is 10 times more base. Going to 5 is 100 times more base, Going to 6 is 1000 times more base, going to 7 is 10,000 times more H+ ions changes from pH of 2. So we are about 10,700 times more base with just the blood.
So you can see how dangerous this time of menses can be for a woman living in a tent in the wild with no medical at all. This was how it was for the Jews wandering in the wilderness.
Now if you add Sperm of 8.0 pH you are in the danger zone of serious problems. The extra amount of fluid at extremely "high pH" compared to the vagina added to the blood of 7.4 pH is a very bad thing to do. Now you are adding a volume of extra fluid at 100,000 times more H+ ions base than pH of 3.
And if a woman were to have sex during this time any bacteria on the penis or on the outside of the area, would take root immanently and she would get sick and die. This is why they checked the women for bad smells after one week and then they were allowed to have sex with their husband.
If women did this today the GYN's would be less than 1/2 the number they are today.
Today the treatment for bacterial vaginosis is to flush with vinegar (lower the pH), infuse cultured flora and give antibiotics to kill the infection; apply an antibacterial cream or whatever the doctor likes to do.
If she were to live and were to get pregnant with no protective flora at the time of conception the zygote stands a HUGE chance of being mutated. The way viruses destroy fetal development is clear and obvious as found in medical journals on this subject.