I just can't believe that such a common, timeless practice such as masturbation, if it was indeed a sin, would not be mentioned specifically in the Bible. If it was a sin they would have mentioned it! As it is, it's not talked about at all.
Onan was not masturbating. He was using the withdrawal method so he wouldn't get whoever he was having sex with pregnant. God wanted him to get whoever he was having sex with pregnant, so he disobeyed God by not [bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse] inside her. The reason why God was angry was because Onan deliberately disobeyed his wishes.
I can think of one reason. The bible was written by men was it not? The Apostles were men, were they not?
Imagine some thing like this, Peter, Paul, James, Luke, Matthew, and Mark and other Apostles too are discussing what they should be addressing to new Christians..
Ok guys, lets make a list shall we.
1. Fornication
all in favor say aye!(every one says aye at once)
2. Homosexuality say aye (slowly every one says aye)
Peter, I did not hear you say aye?
Peter: I said aye! You just did not hear me!
Ok then moving along,
3. Worshiping false idols (all ayes!)
4. Baptism (all eyes)
5. masturbating (room is silent)
I said masturbating! (still silent)
Come on guys whats it going to be here?
Peter starts to speak but is interrupted by Matthew, any one else besides Peter?
Luke speaks up, as a physician I see no harm in it, not that I do it, I would never do it, but speaking as a physician only.
Room is silent every one looks around each other.
Ok then perhaps we can come back to this one..