let me spare you some slack and respond like a bilogy teacher. when the d*ck is erect, the muscles compress the urethra duct really tight and the vas deferens, i.e. sperm duct, is free. Please can you try to urinate with a hard erection, i mean outside a pu**y, and see how tight the stream is, at times even uncomfortable. Its a biological way of safe guarding pu**y from UTI infections.