As for making sins with yourself G-d forbid, that's so 2015, modern science has even lead to completely secular groups against masturbation; reduced sexual stamina, feelings of depression, massive body odor, anti-social tendencies and a hundred other problems. The Jewish law of staying in until your foot is no longer hard has a nice physical backing. Of course, we don't really care if there's a scientific explanation, Hashem said "do this and not this" so we do and we don't, the Rebbe said that if there's a physical reason, there's so many lofty reasons it burst out here, if we don't understand it means the reasons are all profound. Hashem didn't tell Noach that he must marry his aunt on his father's side, so we can ban it, but because it is permitted, we are careful to strictly avoid seclusion with said person.