I am not talking about CSA, I am talking about video material of CSA. Most countries with marriage ages that low have much more wide-spread bans on videos including sex of any kind.

As for prosecution, yes, it is still illegal if it is not prosecuted. There are many reasons not to prosecute something ranging all the way from resource and other means related concerns to intentionally turning a blind eye and only a small minority of them would lead that country to actively sabotage a major international investigation, especially after the trade-offs are considered (such as loss of international reputation by refusing to cooperate).