sexual surrogacy

It's funny, in a way, to read in the NYtimes about how conservative the French are, as they allow prostitution but not sexual surrogacy...(see Disabled People Say They, Too, Want a Sex Life, and Seek Help in Attaining It)

What is repugnant and what not is still quite a mystery to me. Recently I was at a discussion group, in the US, where with lots of head nodding it was agreed that obviously, prostitution should be illegal, while paying for egg donors is not (kidney donors are of course a different matter).

While there was a recent paper on "Does Legalized Prostitution Increase Human 
Trafficking?" by Seo-Young Cho, Axel Dreher and Eric Neumayer


The abstract reads:

"This paper investigates the impact of legalized prostitution on human trafficking inflows.
According to economic theory, there are two opposing effects of unknown magnitude. The
scale effect of legalized prostitution leads to an expansion of the prostitution market,
increasing human trafficking, while the substitution effect reduces demand for trafficked
women as legal prostitutes are favored over trafficked ones. Our empirical analysis for a
cross-section of up to 150 countries shows that the scale effect dominates the substitution
effect. On average, countries where prostitution is legal experience larger reported human
trafficking inflows."

I haven't read it, but, let's assume it is true. How about other aspects that may correlate (maybe negatively) with legality of prostitution like violence? And is it obvious that sexual trafficking would be reduced, not just shifted around, if it became illegal in the countries it is legal now?