Okay, hold up, let's stop spreading disinformation here.

>policy finally caught up to them

The generation where the gap was the largest, after the introduction of the one child policy(50ish years ago) and before massive rapid orderly urbanisation which decreased the relative economic advantage of having boys(30ish years ago), has already "passed". You do not see sex number gaps in Chinese cities anymore than you see them in western european cities. Where the gap is present is in impoverished rural areas, but even then the bulk of that has "passed"; especially in those areas, the expectation is marriage at 25ish for men, men who are now 40ish.

>so that fuels a huge sex slave trade

China does not have a huge sex slavery problem. According to the walk free foundation, focused on modern slavery, less than 0.1 percent of the Chinese population is enslaved. That number is comparable to or better than Eastern European EU members, who generally are the best of the middle income developing countries in most things human rights. Compared to other developing middle income countries, China is actually very good at combating modern day slavery.

What China does have is a prostitution "problem". Prostitution is illegal in China, but tolerated. That system means that sex workers don't have the legal protections they should have. That is a million miles away from the situation you suggest is present.