My Dad has his own bathroom in his room, its very nice nothing wrong about it. Yet still He uses my bathroom, but thats not all.
He uses it in the middle of the night. Ill wake up and here him pissing at like 3 in the morning. He also Sometimes takes a HUGE dump in there and dosnt flush just leaves it there. I think thats at least alittle insulting,
100% Not normal!
See what he says.
If that doesn't work start using his in the middle of the night. I'll bet he'll get the message pretty quickly. Make sure you're noisy so you wake him up.