This interpretation that the Ibn Ezra mentions and rejects is an interpretation of the simple meaning (p'shat) of the verse (Vayikra 18:22). It is likely that the Ibn Ezra rejects this as the simple meaning of the verse because it's doesn't make sense in the context of Vayikra 20:13, which explicitly refers to two males.
Regardless, for halachic purposes, we are concerned with the exegesis of the verse, not the simple interpretation. Exegetically, the Talmud derives from the phrasing "ואיש אשר ישכב את-זכר" (Vayikra 20:13, "And a man who lies with a male...") that the violation involves a man engaging in relations with another male of any age (Sanhedrin 54a). The Talmud explains that the term זכר ("male") cannot be a specific reference to hermaphrodites (Chagiga 4a), even though this particular biblical prohibition also applies to a man copulating with a hermaphrodite (Y'vamos 82b).
"משכבי אשה" (Vayikra 20:13, "...the manners of lying with a woman...") is interpreted exegetically as a halachic comparison between ביאה כדרכה and ביאה שלא כדרכה for a woman. As far as simple interpretations of that expression in the context of the verse, see the Ibn Ezra (Vayikra 18:22) and Rashi (ibid., 20:13).