Well you can’t make a hiring decision on that basis in most places unless you have a reason. What constitutes “a reason” being variable. Generally if you are prohibited from making a decision on a certain factor, you may not ask about it during an interview.

Sex discrimination can be constituted by various things. For example asking about maritial status, children, plans for pregnancy, soliciting sexual favors, etc. Also in some places, if you thought someone might be trans, you could not ask them about that.