I'm courious about this because I'm looking into becoming a landlord and I've been researching the housing laws.
In your example, if you get a lease on an apartment and you are the only one on the lease, then ONLY you can live there. Most landlords have problems when a tenent starts moving people into the apartment without adding that person to the lease.
There are many reasons why that is a problem for the landlord... and it doesn't matter the sex of the "new" tenent... for one, there is more wear-and-tear on a property when more people live there. There is also a higher probablility of liability for injury. That is why almost all landlords charge more money when there are additional people living in a unit.
When some asshole moves someone in and doesn't tell the landlord and/or doesn't add them to the lease, then the LL doesn't have any legal recorse for damage/loss of rents/or lease violations against the "new" tenent.
Remember, the LL is not leasing out an apartment to be a nice guy/girl.... they accept huge responsibility/liability for investment purposes...... when someone is living in THIER property without a lease, then they need to/must either get them on the lease or evict them.
So I ask you, in your example.... was the "boyfriend" added to the lease when you tried to move him in?