Why? It's not exactly a stretch to day that pornography and prostitution are clear signs of the degradation of a society.
The point is that sexuality is a natural part of our life so IMO we have to free is as much as we can instead of trying the classic Victorian style. The same is for alcohol, drugs (both legal and illegal), ...
Only switching from a somewhat sexual repressed country like Italy to more free countries like Sweden before, French now I see a big improvement in mean people satisfactions and awareness, Italian's watch far more porn and goes far more with prostitutes than Frenches or Swedish, just as an example.
I see child being different in more free countries than in others, for instance in Italy and UK I see children that are in medium like earthquakes, in Sweden and French they are far more calm again in medium but in Italy and UK parent's "watch and supervise" far more their children than Sweden or France.
long story short, use education, culture, not stick nor carrots to build a better world.
"Prostitution is a sign of the degradation of society" is a claim that requires evidence. I'm happy to hear whatever evidence you have. "Lots of other people think so" is not evidence.
Given that prostitution has existed in pretty much every historical culture we've been able to study, I'd like to know when and where the society that they all degraded from can be found.
(Incidentally, there's plenty of historical pornography as well, dating back at least to Rome IIRC, although of course it didn't really blow up until photographic reproduction became cheap and easy.)
Migrant labor is not a sign of degradation either, but it leads to many to disrespect others in increasing fashions, where men are more likely to abuse their power over others.
That's irrelevant to the claim "prostitution is a sign of the degradation of society."
I'll assume every morality is invalid. Instead base ones intent on the harms their actions cause.
Assessment of harms depends on a value system; you can't escape the need to choose a morality.
When your choice of morality affects more then a few close people, it is by definition unjust.