This halacha pertains to a most unusual case, a woman for whom sexual relations trigger uterine bleeding. The logic of the law is that if relations cause a woman to become niddah, a couple will never be able to have relations.

A woman who has a wound or lesion or other medical condition that causes bleeding or who is expecting her period does not fall under this law. This law only applies if bleeding is found very shortly after relations and only if the three times are consecutive.

Any time a woman becomes niddah during or right after relations she should contact a halachic authority, because with good advice she will prove herself not subject to this law. Additionally, couples can take simple steps such as using colored sheets, waiting a while after relations to clean themselves, cleaning themselves with colored materials, and not looking deliberately for blood to minimize the chances of any related question arising even once.

Please write back with any further questions.