The Rambam that says that a Mikva doesn't make us pure while we immerse ourselves, but rather when we come out of it, we are purified.
I think if we flip that idea over, what is metamei us?
Not masturbating itself, but it's what we do afterwards that's really metamei us.
Once we did the aveira, it's already done. Now what?
Do we brush the dirt off and pick ourselves up or do we go deeper into it, since we're already so lost we don't even stand a chance.If we pick ourselves right up, the aveira will hopefully not metamei us nearly as much.