I don't see a problem either, both are consenting adults who enter into an agreement with free will, if the person offering favours doesn't wanna do it anymore then they move out it's that simple. It's a mutual relationship, particularly for guys they'd buy a woman gifts and stuff and he'd probably get sex in return, its not much different. The person providing a free rent room recieves sexual favours instead of rent money. For all intense and purposes peanuts could be used as the currency, it's only the fact the cash is the universal recognised form of currency that it is used 99.8% of the time.