Hehahah ... What a sad plan.
I think that if they wanted to protect their minors, they would've done it in the first place. Banning adult/violence oriented works now won't help them because there are always ways to get your hands on adult content magazines... especially in japan... Also, the law should concern the future mangas, the ones that are just starting not the ones that have a large fanbase.
First, making the online scans illegal and now censoring the scenes that make manga manga. :S I don't understand these people... If you are against it don't watch/read it and you can always raise your kids the way you want
Adult magazines wont be affected. It's just that it will become really sad to watch/read the fighting scenes in the shounen series.