Here's the facts:
- Masturbation is built upon Sensual desire. Good luck having a beat while considering how best to mow the lawn and hang out the washing.
- Sensual desire is a hinderance to practise.
- Kamma-Vipaka has it that a volitional action in the present will have an effect in the future. This is how habits are formed.
Nobody is going to burn in hell for having a beat, nor for having appropriate sexual relations with their partner - But it doesn't lead to Nibbana. Sensual desire is a hinderance.
"For a disciple who has conviction in the Teacher's message & lives to penetrate it, what accords with the Dhamma is this:
'The Blessed One is the Teacher, I am a disciple. He is the one who knows, not I." -