I really don't think you know how to read, but I'll toss my hat in the ring.

What people mean when they say they'll fight, they mean because of ~unknown danger~

If someone puts a bucket on your head and you're blinded, you do not, I repeat, DO NOT KNOW THEIR INTENTIONS. Some people will immediately defend themselves, and start swinging. Others will have severe panic or fear. No one just goes "oh goodness me, is that a child placing a bucket on my head! I've been had!"

If that was the case the title of this TIFU would be "my girlfriend MISUNDERSTOOD why I didn't want to take a shower " not use a lame emoji to reference periods. He's immature.

He's made it clear in this post that he's grossed out (or uneducated) by it.

Except based on how he's typing (ex. Pictured himself standing in bloody water, "not happy about it", "womanly shit", and he knew she was upset about getting her period) it's clear he's somewhat repulsed by periods (or at the very least uneducated). He needs to grow up or at least communicate with his gf properly.

Periods make women really emotional, they can even make you bloated or feel unattractive, you basically shot her down over something she has no control over. Also you have no idea how periods work (when it comes to showering?? Do you just think it's a constant river???) If you don't know, ask her (and be polite) women appreciate when men take the time to learn about periods.