Personally, I find it kinda hard to have a concrete stance on this. Too many ways to see the issue and many more answers. None of them fully right. That said, here is what I think -
1. Because of regional, societal and cultural influences. People of some regions are more liberal in that regard. In some places, the society is less tolerant of prostitution. Their legislatures reflect their views.
2. I would not want any girl to be forced to sell her body. I say "forced" because 90% of the time, girls enter into prostitution out of economic compulsions (I personally find it below human dignity to do it for any reason, even for money, especially there are other alternatives available, admittingly less lucrative but more dignified like manual labour). At least that's what I think happens here in my country.
But will making prostitution illegal make things better for such women? Optimist in me hopes that it'd but my cynical side tells it won't. It will certainly reduce the amount of prostitutes but it won't put an end to it. Things would more or less stay the same.
The lesser evil, in my opinion, is then to keep prostitution (being defined as "voluntary exchange of money for sex") legal but with some special rights afforded to prostitutes so that they are not exploited against their wish; some sort of protection for them. Atleast. it would be better than over-exploitation of women with no-one to turn to to get their rights enforced, in case prostitution is made a criminal offence.
3. Sex isn't indecent. It is a function of creation, expression of love, etc. Paying for it ,however, changes the context - you're satisfying lust and that too without any emotional attachment with your partner.
And pay in what way? By buying them gifts? Well, if one is doing it only for sex - not really an ideal boyfriend/husband.
That kind of thinking portrays women as "objects" to satisfy lust. I find it morally objectionable.
Moreover, there is great difference between the emotions, the context, etc in paying money in case of prostitution and spending money to keep your lady love happy and any sex resulting to due to it.
In a nutshell - I don't like prostitution. To be honest, I doubt those in the profession like it as well. Who does not want to live a dignified life? Bar a few, most of the women/girls are forced to give up their self respect (prostitution involves lost of dignity, in one way or the other) due to economic compulsions.