Sforno on Leviticus 19:20/ספורנו על ויקרא י״ט:כ׳

1It is a fact that most of the statutes listed appear to forbid only things which appear opposed to what we are familiar with as “laws of nature.” The prominent exception to this is the legislation involving the שפחה הנחרפת לאיש. Surely, the fact that the male partner does not become subject to the penalty of 39 strokes whereas the female partner does, appears hard to understand and to reconcile with what logic dictates. Furthermore, the fact that the male partner in the sin is required to offer a guilt offering, איל אשם, although he committed the sin knowingly, seems completely at variance with what we know about the function of such offerings. Nonetheless, the Torah herself provides a partial answer to this unusual legislation by writing כי לא חופשה, his sin is viewed as somewhat less serious seeing that she had not been legally capable of entering a binding marriage, the requisite financial token signifying her betrothal to the man being legally meaningless as she could not “own” anything, still being enslaved to one of her former masters. His principal sin consists of desecrating the institution of marriage, a Divine institution, hence a desecration of G’d’s name when ignored. Engaging in sexual intercourse with a woman who is still partially a slave is a form of practicing promiscuity. [my words. Ed.] The scenario described by the Torah denigrates in the first instance the dignity of the active partner, the male, not that of the passive partner the female. It is most likely, that she, the “passive” partner had seduced him and is therefore deserving of the penalty decreed for her by the Torah. The deliberate nature of the sin, as far as the male is concerned, is the fact that he knowingly demeaned himself. This makes it akin to an inadvertent sin, qualifying for the guilt offering decreed.