There is a concept in Halacha, which originates in the Gemoro, of Uman BaAvidtai Torid, that a professional who is involved in his work need not be concerned that he will have inappropriate thoughts arising from his work. For example, when a person is involved in causing two animals to mate, we are not concerned that he will have inappropriate thoughts. Uman BaAvidtai Torid, however, is not a blanket rule to be used in all situations. For example, if a girl wants her hair cut by a professional male hairdresser, or a woman wants her sheitel cut by a male, this is unacceptable because the man is involved in actually beautifying the woman. In such a case, we do not use the rule of Uman BaAvidtai Torid; we don't say that since he is involved in his work he will not have inappropriate thoughts. Likewise, the Rogatchover Gaon says that in regards to women singing, such as girls singing on a bus, we do not say that while the male bus driver is involved in his work, he will not hear the singing. Consequently, that is not acceptable.