He could have just had sex with her at will, but he is trying to run off with her. If she is willing to run off with him she would definitely have sex with him. If that's all he wanted he could have just had sex with her at school and gone home to his wife. It is the act of leaving his life behind that shows he wants her for more than sex, he obviously wants to be with her. I am not saying she is mature and capable of making decisions or that her parents don't have a right to be angry. Remember though it used to be common practice for teenagers to get married.
I think its romantic that a professional man in the twenty first century would sacrifice his career and marriage and run off with someone he loved defying law and popular attitudes. Love is not rational it is supposed to conquer all or so we have been told.