A little weird?

I asked my Family Law class what policy reasons might there be to prohibit marriage between a woman and her adoptive dad (she was 15 when she was adopted, but 21 when she started a relationship with him, leading actually to her being pregnant by him when she was 22). Except for two or three, most thought such a marriage should be permitted (though through a legal maneuver of first vacating the adoption). Interestingly, they believed that parenting stops at 18 [really??] and therefore he was no longer fulfilling his parental duties.
I reminded them that when Woody Allen wanted to marry Mia Farrow’s adopted daughter (Woody and Mia had been in a pretty stable relationship for years), there was almost complete public condemnation. Someone said “hey, that’s a little weird,” but I wasn’t sure whether they were referring to Woody’s choice of bride or the public condemnation.