A High Priest is only permitted to marry a virgin, one who has not had sexual relations. The scholars asked Ben Zoma: A virgin who is pregnant — is a High Priest forbidden to marry her? What is the issue in doubt? On one hand, she might have had sexual relations and yet be a virgin, such as if she was impregnated by a man who is expert in having relations without tearing the hymen. Shmuel, one of the greatest Babylonian sages, bragged that he was expert in having sexual relations without tearing his partner’s hymen. On the other hand, she might have become pregnant without any sexual contact at all, and then she would be permitted to a High Priest.
Ben Zoma responded that she is permitted to a High Priest since only a very small number of men are expert in having relations without tearing the woman’s hymen, so we must suppose that she became pregnant without sexual contact. Her pregnancy probably occurred when she bathed, from sperm left behind by a man who had bathed in those waters before her.
(Babylonian Talmud, Tractate Chagigah 14b)