I answered that point dear.
With contraceptive technology available- s*x and child bearing are two different issues. In fact now they are not related to each other. Sex is not going to result in child bearing if contraceptive is used- and s*x is not required for child bearing (test tube baby). Thus, free exchange in s*x will not result in child bearing.
In fact legalising s*x trade will reduce/eliminate child bearing in trade of s*x, due to better training of workers, better working conditions, better social welfare measure, may be insurance against child bearing in case of contraceptive failure.
Thus your concerns are better served in legalised prostitution scenario.