"You don't throw up your hands and effectively say "well bad things are going to happen anyway, they've always happened, so hey, let's just decriminalize those bad things, problem swept under carpet, phew."
Where on earth did I say that? 
All I am saying is that changing the age of consent for marriage in Britain won't necessarily fix the problem. Even community outreach won't necessarily fix the problem.
There was a documentary shown a few years ago, made by a journalist who visited families with children with congenital health problems in these communities, and those parents and grandparents categorically refused to believe cousin marriage was the problem, saying things like "we are cousins and our children were fine." One family believed it was British doctors causing the illnesses.
I have an old college friend, lovely chap. He's educated, he's got a professional job, born and brought up in Britain. He married his cousin in an arranged marriage. When I asked him whether or not he was concerned that his children or grandchildren may have problems, his response was "well, what can I do?" He couldn't go against his family's wishes; it would have been anathema for him to do so.
Look at the drivers. A lot of cousin and child marriage is to do with keeping wealth within families, ensuring a daughter-in-law is in a safe environment with a familial connection to her mother-in-law, and immigration rights.
Changing the age of marriage consent does not address any of those drivers, and those drivers are more powerful for families than an age of consent law.