The post-WWII marriage boom (followed by the Baby Boom) may have shifted some boarding house singles into private apartments with mod. cons. too. My impression is that married couples seldom lived in boarding houses; they waited to marry till they could afford homes of their own.

I wouldn't be surprised if some boarding houses had a 'no babies / no couples' policy; perhaps as much for respectability (marrying before you could afford your own home suggested poor impulse control) as for noise.