The Vulgate translation of Ezechiel XXIII:19 and 20 is as follows:

19.  Multiplicavit enim fornicationes suas, recordans dies adolescentiae suae, quibus fornicata est in terre Aegypti.

20.  Et insanivit libidine super concubitum eorum, quorum carnes sunt ut carnes asinorum: et sicut fluxus equorum fluxux eorum.

Literally this translates something as follows:  She multiplied her fornications (rather "acts of fornication", perhaps?), remembering the days of her adolescence, when she committed fornication in the land of Egypt.  And she hungered for her lovers, the flesh of whom was like the flesh of donkeys, and the ejaculation of whom was like the ejaculation of steeds.

Cicero uses the word caro (= flesh, from which carnes) in a derogatory fashion to refer contemptuously to a man, so literally "that lump of meat" or "flesh".  From that it requires a small leap to get to the male member, and the implication obviously from the particular verse is that the lovers were hung like donkeys . . . Pliny uses the same word for the pulpy parts of fruit - hence again perhaps the connexion with seed?