Of course, most people find practicing sex during menstruation unclean and unhygienic and then again, there is almost half of population who do practice sex during menstruation because they believe it is the safest way to avoid unwanted pregnancy.
Sex during menstruation is not 100% safe practice for those who want to avoid unwanted pregnancy. Surprised? It is possible to avoid unwanted pregnancy if a woman and a man follow the menstruation cycle calendar, but there's no other alternative to using contraceptives such as condoms in order to avoid pregnancy and other infections.
And there is another scary fact the possibility of being infected (not only uterus infection) but hepatitis, HIV or with any other STD is high not higher than it would be if woman wouldn't have menstrual bleeding, but just as high.
On the other hand, some gynecologists claim that there is a higher chance of getting STDs indulging in sex during menstruation. STD do not spread only through infected partners but the cervical tissue can become very sensitive during women's menstruation, so even the smallest dirt particles in the air and the pollution around one can invite STDs.
Additionally, the chance of getting uterus infection is higher than normal. Here is a reason why: the mouth of uterus in often wide open during the period which is the main risk of being contaminated with different kinds of uterus infections.
This is the main reason possibility if infection is higher than during the rest of the month, so it is better to avoid contact. Another reason is that female pelvic tissue is extremely soft during menstrual bleeding, so the physical intercourse could cause injury, which could result in infection.