The evidence is overwhelming that the Bible condemns as a moral offense, not just a matter of ritual purity, sex during menstruation. Early Catholic authorities have seen it as a major sin, others as a venial violation. Christian writers in the middle ages all condemned sex during menstruation. They varied into how seriously they saw it. The more common opinion was that sex during menstruation was a mortal sin.
In 1951 the Catholic Church nuanced its position on the issue during advocacy of the rhythm method of contraception. But the practice has not been specifically condoned, although there is no contemporary condemnation of it within Catholicism.