This forces me to think through more on this issue. The questions that arise are: why would being dirty be sinful? I know that God doesn't force us to sin... so why then is menstruating need a sin offering? That CERTAINLY isn't something I choose to do. Maybe it is in reference to the original sin?
Gen 3:16 - "To the woman He said: 'I will greatly multiply your sorrow and your conception;...'"
I know that sin offerings were offered up routinely until the power of Christ's death deemed the routine of it unnecessary. Perhaps after she menstruated was the time that she offered up the sin sacrafice for the original sin..... But then what of the man's discharge? I have no answer that coincides with the theology of why the woman would offer a sin sacrafice.