Is it unethical to ask for homework help on StackExchange?
This might depend on your university's policy on cheating and plagiarism.

For our university (though the precise details may vary from course to course), you are allowed to use StackExchange to help you to understand concepts, but you are not supposed to use it to help you solve assignment questions. 

We tell the students that if there is any risk that they may have read an answer or some code or whatever that might have influenced their answer, then they need to cite it. With a proper citation, they cannot be accused of plagiarism. We may, however, ask the student to answer extra questions in such a case, in order to demonstrate their knowledge.