This is just purely ex post facto pseudo-scientific rationalism of rampant misogyny. If you want to be true to the statements made in historical sources (e.g., the bible), try to put yourself in their shoes -- in this case, imagine that you are a patriarch in a deeply misogynistic society that views women and children as property and not people. The reason for the law is thus completely justified to such a person: to them, menstrual fluid is gross and just another example of why women clearly do not have the full rights of a "real person".
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.
It is interesting that we hate cynicism when expressed by others yet we feel entitled to express it ourselves. Sometimes it is a good idea, when judging the behaviour of others, to consider options other than malice.
On the other hand, a broken clock is right twice a day.
A lot of the passages that seem completely controversial now, can be understood if you think about what was going on.
For example, saw a bunch of people saying it was proof of misoginy the fact the bible consider menstrual fluids "unclean", thing is, that law was enacted when they lived in a mobile tent camp, if a person with a blood-transmissible disease washed on the middle of the camp water source for example, it could quickly infect everyone.
Or Paul in New Testament saying men should NOT cover their heads, make a lot of sense when you find out people at the time decided to adopt a fashion from some roman nobles that started to use robes with head covering during religious events, with the intention of appearing more spiritual.