If men are only with woman for the sex (what they commonly say), and women are only with men because of money (what I hear from men).. Why is prostitution commonly illegal/banned?

There is a perfectly logical explanation for that:

You brought the concept of 'legalization'.

If the gov't was able to tax the process, then it would probably be legal.
If the gov't can't get a piece of this transaction.. why allow it for free?

-MS2

heh dumb government getting in the way of things.