You are ignoring the very important element of poverty. The prostitutes in these countries (SE Asia, historically Eastern Europe) are typically desperately poor and these tourists are burning money on leveraging the wealth they have from the imperial core to basically use these people as slaves, typically with few words that even could be spoken between then and no recourse for the prostitute if the john gets violent (what is she going to do, tell the cops that she was assaulted by the westerner she was prostituting for? Whose side would they take?)