There is absolutely no truth to the rumour that sex cures menstrual pains.
Menstrual blood is a rich environment in which bacteria and viruses can grow very quickly.
If a menstruating female has sex with a male who has an STI, she is more likely to get infected than during other times of her monthly cycle. In addition, she is likely to get a more serious infection when she is menstruating. This is because the opening of the cervix is wider than usual so that the menstrual blood can flow out. The STI germs can travel up into the uterus and fallopian tubes and can cause an infection high up in the female reproductive organs. This can result in infertility.