Males may marry outside of Islam but it is expected that the new wife would convert pretty quickly as it would be very hard to maintain a solid marriage between a practicing Muslim and a wife who does not believe. Female Muslims are forbidden to marry outside of Islam because the Qur'an says the man is the head of the home and the wife is to follow him, if he is not Muslim then she would be outside of her religion. If he converted to Islam, he would then be allowed to ask her family for her hand.