The following is a bit personal, but perhaps your wife has experienced something similar.

Personally -as a male- I always feel a bit embarrassed when buying toilet paper. I know it's just a normal body function, but I still don't like it. I think it has to something with my early youth.

When I was a child my parents had a hard time to learning me to become potty-trained. At some point my mother got sick of it and from what I remember she pushed my face into my own feces. From that moment on I was potty-trained.

I believe a side effect of this experience was that I'm annoyed by defecating myself and I also am really annoyed by for example people farting. It's probably 1 reason why I don't want to have kids (would hate the baby phase), though I would not mind adopting a kid that's potty-trained.

Perhaps your wife has had some traumatic experience related to her menstruation causing this annoyance.