Corrected entry: When Jack is introduced to Mr. Roper for the first time, the girls just say he's their landlord (no name given yet), however after Jack's talk in the kitchen with Chrissy (in which still no landlord name given), Jack storms out and starts out shouting "Look here Mr. Roper..." He still should not have known the landlord's name at this point.
Corrected entry: Janet mentions that Roper's rule of living in the building is no pets or babies allowed. Pets is one thing which is common in many apartments, but it would be unlawful to not allow babies in an apartment building.
Correction: While it would be illegal, landlords sometimes do illegal things, such as discriminate. It would also be illegal for him to deny renting to a straight man living with two women. But, Mr. Roper has been renting a long time and it wasn't until the Fair Housing Act of 1968 that made it illegal to deny renting to people with children (so less than a decade ago when the show was made). Not only may Mr. Roper be unaware of the law, but potential tenants may also be unaware of the new laws and just accept his terms. It would only be a mistake if a court ruled that Mr. Roper was acting legally.
Correction: They could have told him Roper's name earlier in the episode, off-screen.