I just checked the zoning regulations of Atlanta, Dearborn and Detroit and found no such law - and I already know there is no Georgia or Michigan code section either. (perk of being a lawyer - occasional free Westlaw access) I'd really love it if either of y'all had a cite you could share... I'd love to be able to prove
snopes.com wrong, but they never are.
As has been mentioned upthread, and from my Property caselaw knowledge, there are restrictions in some localities on how many unrelated people can live together - but not because they're "brothels." That designation is a criminal one, and is only determined by the activities inside the house, not how many people live there.