Surrogacy vs. Prostitution

Can someone explain to me how it is legal for a perfectly healthy woman to have children via a surrogate yet prostitution is illegal?

A woman or a couple can pay another women to bear a child for them - because they don't want to deal with the trappings of pregnancy (see Sarah Jessica Parker) - and it's all nice and legal. But purchasing sex is illegal.

I understand the logic behind prostitution being illegal.

I get it.

But how can anyone make the argument for the legitimacy of surrogate motherhood in the same breath? How is one OK and the other not?

So you can purchase the end result of the deal, which is a lifetime committment, but you'll be fined and ridculed if caught trying to buy a booty call.

I ain't see'n the logic here.