Immediately.  This point cannot be emphasized strongly enough.  You have the right to refrain from answering questions without a lawyer and asking to speak to a defense lawyer is in no way an admission of guilt—it is simply your legal right.  Even if you whole-heartedly believe that you are innocent and want to cooperate in law enforcement’s investigation into a sex crime, you should still speak to a lawyer first. This is because you have no way of knowing what type of evidence the prosecution has already obtained, and you could inadvertently make statements that could damage your case.