- It's important for a doctor to be good at listening to his patients.
- Sometimes a manager needs to make decisions that will be unpopular with his staff.
- A nurse knows that she has a lot of responsibility for her patients' welfare.
- If someone wants to become a scientist, he needs to study hard at school.
Answer: It depends what you mean by 'wrong'. Grammatically, they're all fine. However, to many English-speakers, there's something just a bit... uncomfortable... about them.
Simply put: why are the doctor, the manager and the scientist automatically men, while the nurse is a woman? Aren't women doctors too? And men nurses? In English it's now customary to include both genders in statements like these - it's polite, even!