Questions: Religion & Spirituality

There's this belief in my religion that a woman mustn't enter a temple (to do anything - pray, etc) when she's on her period as it (the period) makes her impure and dirty. My mother supports the belief 100%, meanwhile I think that the belief is ridiculous and oppressive.
She said that, "Would anyone like it if a dirty person who wore dirty clothes came in the temple? It's the same for a woman on her period. Blood is coming out from her vagina, which can spoil, ruin the floor, so the rule was made that a menstruating woman must be prohibited from entering the temple."
There's also a belief that a woman mustn't touch, you know, grains and other things. She had to get help in taking some wheat out of the box from me just because she was on her period, for the same reason - period makes women impure and dirty.
I had asked, "Our blood doesn't pour in the grains, does it?"
"But our shadow does."
So, even the shadow of menstruating women is bad!!! ?


If I am correct, how can I convince her?