Just like the ol' "13 is legal" myth, that's not entirely true. If you are found to have child porn, the police will do their best to charge you with a related charge (generally if it's with a local child, that'll be coercing them into taking photographs). They cannot charge for simple possession due to poorly worded laws and concerns that doing so will open floodgates for innocent people to be wrongly accused of possessing child pornography -- such as taking a photo of your young child in just a nappy/playing in the bath, etc. But make no mistake, if you're found with child porn, you'll enjoy up to 23 days in custody at a police station, while all manner of confessions are coerced out of you, before then going on to be formally charged.

Likewise, the "13 is legal" myth comes from the fact that the national age of consent is set at 13, but that is superseded by local laws which put the actual age of consent at 18.

There is just one major exception to that though; teachers. They basically get away with anything. Primarily because their local education boards do their best to prevent such matters being reported to the police. They do the same with other issues such as bullying too mind -- which is causing a major uproar right now due to related suicides. It doesn't help that the police basically won't pursue any reported crime for which they feel they cannot get a prosecution (confession) for. But that's another topic entirely.

So is Kojima a pervert? Probably not. He knows what sells in the local market. Sex isn't currently* a big taboo here unlike in the US. Boobies are not considered evil. Now, if you want to think about being perverse... you could always ask a Japanese person to show you "kanchou".

* That is until a certain porn novel writing governor of Tokyo gets his way, then sex will be taboo if it doesn't involve live action.