One must be really a hopeless ugly German to pay a woman from EE for sex. They hope for a relationship anchoring them in the country and need a guide through the ruthless rules and bureaucracy, you receive sex free of charge. Bulgaria, Romania, Moldavia, Poland, Belarus, Ukraine - you pick the flavour. Historical justice... not.
> One must be really a hopeless ugly German to pay a woman from EE for sex.
Not really, if you are interested in obtaining paid sex you don't have much choice. Most of the brothels are staffed by EE sex workers, been that way for well over a decade. German sex workers are rare in brothels, they work mostly escort and camgirl stuff.
The problem at its core is extreme poverty in EE - not many job opportunities that pay decent wages, so many people from there come to the nearest wealthy country aka Germany. Men go on the construction/agriculture gig job scene, women to prostitution... it doesn't pay much and the conditions both of work and living are extremely bad (just google "Arbeiterstrich München" if you want newspaper reports on the construction gig workers), but even this is better than life in their origin countries. As a result, German workers simply can't afford to work in this environment.
Many EE countries are already in EU, their citizens do not need to go through ruthless rules and bureaucracy to live in Germany, they just need to find any work there.