An obvious difference is that it's the semen that makes things unclean, while for menstruation it's the woman herself that is the unclean agent. For semen, the

An obvious difference is that it's the semen that makes things unclean, while for menstruation it's the woman herself that is the unclean agent. For semen, there is a clear distinction that it is anything it touches becomes dirty and needs washing. But for menstruation its whatever the woman touches that becomes unclean (and for seven days it seems!), not merely what the blood touches. The original comment thus provides an explanation for why these two cases are treated separately, which is basically what you asked for.