Reference for Sanhedrin 115:10

אימא אשת אביו היינו באשתו ולא באשת חבירו אימא לאחר מיתה דומיא דאמו מה אמו דלאו אישות אף אביו דלאו אישות

R. Akiba said: His father means 'his father's wife'; his mother is literally meant. And he shall cleave, but not to a male;<span class="x" onmousemove="('comment',' I.e., a prohibition against pederasty. This is deduced from the fact that it is natural only for the opposite sexes to cleave to each other. ');">¹⁰</span>

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