7 Year old boy Masturbates

[snakey11]

This is totally unbelievable to me, that my 7 y/o rubs and twists his penis while he thinks thoughts about his sisters.  I am totally dumfounded.  He says he thinks about their p.p.'s, thats it not doing anything with them but just them.  Only my younger daughters no my oldest.  Also, this is what is most puzzling, my husband and I have been married 17 years, we don't have a TV and my children are homeschooled.  Where in the world could he possibly know what to think and more so why does it turn him on?  I grew up in the 70's with a divorced "free type disco 70's sex loving mom" and we saw it on HBO, Dirty Movies etc, and we never were like my son?? Also in the beginning we thought he had a bladder problem, but the came to find out he wet his pants because of his thoughts and masturbating, he is just to young to ejaculate.  We just don't know what to do as we have other boys that are all witness to this and it is gross to them and they are starting to hate our 7 year old.  HELP what did we do to cause this and what can we do to end it, he is just too yound and I know it is affecting him,

[margypops]

It does sound like a learned behavior so check out who he is with when you arent around, ask him pertinent questions, he will say he doesnt know if you ask outright"Has anyone done this to you" ...How old are his sisters?