Technically, sex with his step daughter would not be incest as there was no genetic connection, morally it might be considered fornication. I’m surprised the arguement was not put forth on those grounds...ala the Woody Allen defense!
Genetically and LEGALLY are 2 different matters. Today, in most, if is not all states, it is only based on the legal relationship! (Unless you're a celebrity with the right connections.)
In a society where divorce and remarriage puts a large percentage of children under the authority of step parents, blood relationships do not form the basis of many/most families. Incest laws today try to protect all the children in a family.
One big question is where the cutoff age should be. Another is whether a step parent should be allowed to start grooming a step child below the age of consent for a sexual relationship once the become of age.
Ohio Rev. Code § 2907.03(A)(5) makes it a crime to “engage in sexual conduct with another, not the spouse of the offender, when . . . [t]he offender is the other person’s natural or adoptive parent, or a stepparent, or guardian, custodian, or person in loco parentis of the other person.”
Incidentally, Lowe did also argue, in addition to his Constitutional claim, that there was a “clear legislative intent to have the law apply to children, not adults,” but that was dismissed by the federal courts as well. I instead focused not on the statute itself, and statutory arguments, but on the Constitutional reasoning used to arrive at the result.