You need to ask?
The answer to all 'how did this happen' biology questions is always 'because those who didn't died out'.
In this case, shagging unconcernedly during menstruation presumably didn't reproduce as efficiently as not. Maybe the unconcerned shaggers passed too much Hep-C around, as suggested. Maybe the abstainers built up so much, er, tension during the downtime that they were really pumping gametes by the time the woman got round to oestrus.
It's all part of Darwin's Master Plan. Howsabout: PMS tends to cause fights between partners. Menstruation tends to keep them going and prevent a standard cure. Fourteen days later the couple are about ready to make up, sweatily, just in time for conception. Works for me.