Nine times out of ten, when a woman says, "I have a boyfriend," what this translates to is, "You just telegraphed too much interest." It has nothing to do with whether she actually has a boyfriend. In fact, if she is attracted to you, she will often deliberately hide the boyfriend from you until after you have had sex with her. Whether she mentions him or not in no way proves that he actually exists—only that she had a motive to mention him. The bottom line is, don't ask about her boyfriend and don't appear fazed if she mentions him. He may not even exist. Just take it as an instance of a lack of interest caused by you telegraphic too much interest far too soon.