Generally speaking, we require a woman to wait three months before getting married when it is necessary to clarify whether she is already carrying another man’s child. According to a baraita quoted on our daf, Rabbi Yehuda also applies this rule to a woman who converts, a woman who is freed from captivity, and a woman who is released from slavery. In all of these cases, Rabbi Yehuda rules that we must be concerned that in their former situations these women engaged in sexual relations – perhaps against their will – and therefore they may be pregnant. We must obligate them to wait three months before they can marry. Rabbi Yosei, on the other hand, permits them to marry immediately.
Rabba explains Rabbi Yosei’s position by saying that since these are all cases where the woman does not want to become pregnant, she will likely use a mokh – a contraceptive resorbent at the entrance of her womb that will keep her from conceiving. However, the woman who was held captive, for example, could not possibly prepare herself in such a way before she was raped?! Rather, Rabbi Yosei relies on the fact that the women turned themselves over in order to keep the semen from fully entering, so we need not be concerned that they became pregnant. Rabbi Yehuda is concerned that they do not turn themselves over well enough, and they may become pregnant anyway, their best efforts notwithstanding.
From a medical point of view, “turning over,” as described, would have no effect whatsoever on the chances of pregnancy. It is likely that Rabbi Yosei’s intention is not simply for the women to turn over, but to take all preparations possible in order to avoid pregnancy, including herbs, drugs and so on. Rabbi Yehuda’s concern is that there are few methods that can be used after sexual relations are over that can guarantee that no pregnancy will result.