GEN1478 One who fondles children, [i.e., leading to his arousal and the ejaculation of semen, cf. Nidah 13b), or masturbates, using his hand or foot, is punishable with misah bidei Shamayim [Divine capital punishment] (i.e. for wasting his semen).  Our Sages, z”L, said Nidah 13b that his punishment is equal to that of the Generation of the Flood, which corrupted its ways. Similarly, one who performs a deed such as that of Er and Onan – “to thresh within and ejaculate without,” so as to waste semen – is sentenced to death, as the pasuk says [this verse]. The pasuk also says concerning those who waste semen Isaiah 57:5, “You who stimulate yourselves under the elah-trees, under every luxuriant tree – [you] who slaughter the children [by wasting semen, he has slaughtered the children that would have been born of that seed. Maharsha, Nidah 13a).  GATES 285-6