It wasn't necessarily 'fighting sexual promiscuity' though. Being raped was enough to put a girl into the clutches of the M(agdalene)S(isters), as was being somewhat precocious. In other words, as so often it was always the woman's fault even when she was actually the victim. Also, and this comes out in interviews with victims of the MS-system which can be found on Youtube, just having an unhappy home situation could find the MS being 'helpful' and offering to take a girl in so that she could concentrate on her schooling - which then however only consisted of hard graft in the laundry.
As to the film- very intense, very moving and above all shocking.