Where are you getting this from?
Well, forget for a minute that it is totally against the principles of Islam to legalise the exploitation of women and selling their bodies. Let's look at it from a totally secular viewpoint. They have legalised prostitution in Europe, in the Netherlands. All it has done is legalise the exploitation of women.
Men can freely exploit women, without fear of prosecution. They have women wearing next to nothing, displayed in shop windows like a cattle market. The sex industry in the Netherlands has exploded since legalising prostitution - it now makes up a significant portion of their national economy. Prostitution has increased since legalisation.
They're talking about importing women (in the Netherlands parliament) from overseas to work as sex workers, because there is not enough prostitutes to meet demand. Sex-slave trafficking has increased since they legalised prostitution in the Netherlands. There is also a growing child prostitution market in the Netherlands.
The pimps and gangsters who run these girls have had their lives made much easier. Although, the girls are required to prove that they are self-employed, there are little checks done to ensure that they actually are, or that they are even of minimum age. It is big business, and big money, and no one cares about the women being used as pieces of meat, because, hey, it's legal now.
Some people who support legalisation think along the lines that since it is legal, there will be more regulations, and the women who work as prostitutes are better protected. That's not the case. What do the women who are being used as prostitutes in this legalised system think about it? I'm sure they don't feel any less exploited or safer.
If you're really interested in this issue, read up on the effect of legalising prostitution in the Netherlands.