I really don't see what the fuss is about--there's no reference to any sort of gender roles here. Translated literally, it simply means "she is dirty, he is clean." Perhaps he just showered and she just got home after a long day's work? The fact that some people feel the need to infer something offensive says more about them than anything else.
Perhaps people are a bit too sensitive these days? Would people really feel that much better if it said "Ona jest cysta, on jest brudny"? Or would people prefer if every sentence were anodyne and sterile?
Uhhg. You can't go anywhere these days without encountering politically correct social justice warriors. Maybe you all can talk about the offensive nature of DuoLingo at the next Bernie Sanders rally.
If you feel offended from this example i think you have serious issues and reconfigure your life
filthy is more extreme than "brudna", and it's worth noting (again), that "brudna" is mostly about state of physical cleanness , especially when describing people.
Seriously, people? What if this sentence is sbout inanimate objects? Or animals?