"...but the prevalence of gangs doing it is surely a sign of a serious cultural problem that needs to be tackled by community leaders."
Like many aspects of human morality, this is something that should primarily be addressed by parents when their children are growing.
Given that victims of child abuse don't have the time to wait around until a generation of parents get the idea, it is up to our Police services to prosecute child abusers, regardless of the cultural background of the perpetrator. Time and again we hear about Police resources being overstretched, and I'm sure there's a lot of truth in it but it is not the real problem here. The problem is allowing police officers to make decisions about prosecuting serious crimes based on the allocation of resources, rather than on the effect such crimes have on their victims.
Police officers should prosecute criminals, regardless of ethnic groupings or so-called cultural influences. To do that effectively they need society on their side, saying 'do your job and we'll back you up'.
Unfortunately all too often we don't back them up, and we are back where we started. We all know there are ways that we can have a society with less crime in it, but it takes effort, and it requires criminals to understand that they will face justice when they are caught. Any failure to prosecute child abusers is a disgrace; blaming that failure on resource allocations, political implications or anything else other than a lack of evidence is even more of a disgrace.