According to the laws of the Torah, one may buy slaves only if they are not Jewish, for it is written “And as for your male and female slaves whom you may have — from the nations that are around you, from them you may buy male and female slaves” (Leviticus 25:44). The early sages ruled the ways in which slaves are acquired: money, contract, and right of possession.
How does a person acquire a slave through the right of possession? If the slave opens the master’s shoelaces or carries his towels to the bath house or any such actions. A different sage added that if the slave lifted the master he has been acquired through right of possession. The scholars asked: If a slave is acquired by lifting the master, a woman should be acquired as a slave through sexual relations, for during sexual relations the woman lifts the man. Answer: When the slave lifts the master, the slave regrets it and the master enjoys it, but when a woman lifts a man during sex, both enjoy it, so she is not acquired as a slave. The scholars went on to ask: If they had anal sex, the woman does not enjoy the act. Answer: Who said she doesn’t enjoy anal sex? The scholars also answered: There is an accepted rule that there is no difference between vaginal sex and anal sex, so the conclusion is that a woman is not acquired as a slave through sexual relations.
(Babylonian Talmud, Tractate Kiddushin 22b)