YANBU to be disgusted, but I'm not sure it should be illegal (or no more than prostitution) because presumably the woman knows what the score is BEFORE she accepts and can therefore go elsewhere, the same as they could if they were going to be charged extortionately high rent. It's a price for the flat - one is in money, the other in sex. Some people might not have a problem with it (I would though). It's no different to prostitution - instead of cash they get a roof over their head. And it runs the same risks.