Much of reasoning behind rules and regulation is not only a physical but also spiritual. I don't know why such things aren't thought these days any more!
In Islam, everyday we pray, we say "gorbatan illalah" which means "for sake of nearness to Allah". And Allah (swt) says in Quran only through self restraint, fasting, consistence prayers and zirk of him, charity, self purification of oneself we can get closer to him.
Now if you think a little, when you masturbate, you see the spiritual distance from Allah (swt) and that is mainly why it is haram. Nearness to Allah (swt) is the ultimate goal. This is why we Shias follow our Imams. We put them as our role models because they were perfect human beings. By learning from them, being like them, and following them, that is how we near to Allah (swt).
I really don't think we should look for "scientific" reasons for everything because "scientific reasons" could be fawled or corrupt. I have heard from "scientific community" both arguments for or against masturbation but at the end of the day putting trust in Allah (swt) is what a Muslims does;
Successful indeed are the almu-minoon,
23:2
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Who are humble in their prayers,
...
23:5
And who guard their private parts,
23:6
Except before their mates or those whom their right hands possess, for they surely are not blameable,
23:7
But whoever seeks to go beyond that, these are they that exceed the limits;