The ONE thing that it is out and out illegal for employers to ask about is medical information. If you open the door by mentioning any medical conditions, disabilities or medication, they can legally go through the door and discuss it. But they cannot legally ask you anything except whether or not you are capable of doing the job as described.
There are other things, such as age, marital status, family situations (or family planning) and so on, that it is not technically illegal to ask the questions, but it is illegal to use the information in making the hiring decision. For that reason it is VERY unwise for the employer to ask the questions at all, since they would be hard put to prove that they didn't use the information to discriminate. But it is not technically illegal.