You're missing the larger point. Masturbating in itself isn't wrong, it's the lust that accompanies it.
All these things that aren't too serious on their own have broader implications. Same sex is immoral. Drinking alcohol is destroying your own body. Abortion is destroying both your own and your fetus (that being said, the health of the mother always comes before the fetus according to the Bible), and if God made sex for pleasure he would not have put the risk of procreation in it.
And no, it's not as easy as asking for forgiveness, you have to be doing it sincerely, understanding what you did is wrong, and doing it out of love of God, and not fear of Hell.