I was recently told by my Brazilian roommate not to brush my teeth in the sink (I wasn't, I just spit in it) because of cultural reasons. Is there a cultural norm in Brazil / Portugal where this behavior is frowned upon?
Searching for "brushing teeth in kitchen sink" returns hundreds of forum posts about this very topic, spread over several English-speaking countries. This leads me to believe that this is a global custom, rather than just a Brazilian quirk.
On a personal level, the aversion to brushing teeth in the kitchen is explained by several reasons:
It's unusual. Likewise eating in the bathroom is a bit strange.
Different levels of cleanliness between the kitchen and the bathroom.
Watching someone brush teeth is rarely a pleasant sight. That's the reason why Hollywood movies never show people using toothpaste and why everyone brushes with their mouth closed.
No. Some people can find that weird and some can argue it's not appropriate to do personal hygiene in the kitchen but that's no different in Europe or North America IMHO. Just talk it with your roomate and maybe you can reach an agreement. I can guess the need can arise in a small apartment with lots more people than bathrooms. If your room mate continues to disagree, propose to use a bucket.
I live in Brazil and have lived here for more than 30 years in many towns in Northern and Southern regions. Also has traveled a lot and know a bit of the main cities like São Paulo, Rio de Janeiro, Salvador, Curitiba, Porto Alegre, etc.