Yeah, I know, but you have to already have a pretty twisted mind to start dating with what is for all intends and purposes your stepdaughter.
If he were actually a father or father figure to her, I'd definitely agree. But by all accounts (that I've read at least) he wasn't even a part of her life until she was 18, only ever interacted with her at Mia Farrow's urging, and then from those initial interactions became romantically interested. It wasn't a father-daughter thing that became a dating thing, as far as I have seen, which I agree would have been pretty twisted. And she was and is a consenting adult who has been in that relationship, seemingly quite stable, for like 25 years now, a fact that seems ignored whenever Woody Allen is brought up.
Like I said, it's a pretty weird way to meet someone. But it wasn't the stepfather abuse of power or outright child molestation it gets frequently caricatured as. And it's worlds away from Roman Polanski raping an underage girl or Harvey Weinstein sexually assaulting dozens of women.