Because what we're talking about is that in those old musicals there was a subtext, which is to say that the actofsex couldn't be acted out literally on the screen.
Even the actofsex itself is boring to some men unless it is ratcheted up in some strange, deviant fashion -- all at the expense of the thoroughly humiliated and debased woman.
As for prostitution, Napolitano draws up an interesting situation involving a young male taking a pretty female out to dinner with sex on his mind for later, versus an older male paying a good looking woman to have dinner with him with full knowledge that the actofsex will occur later.