This thread is almost as fun as the one about Rachel Dolelzal! People just keep on tripping over the contradictions of ideology. I think Kalwejt's post-- and I don't mean to call him out in particular or anything-- sums it up best: "there should be a line". You're not really sure where exactly this line should be drawn, but you have the general impression there ought to be one somewhere.
You're right. It's hard to say where exactly this line should be drawn and it would be arrogant to pretend to know every bloody answer. Unfortunately, just stating all over again "consensual/who am I to judge?" it's quite vague as well.
I guess I can address only this particular case. I have serious doubts about "consent" argument applying here, due to whole complicated dynamics of parent-child relationship. And even if we consider it perfectly consensual, there is still a question of longterm psychological effects, which puts "but nobody is getting hurt" argument in doubts. I'd probably agree with most of you if it was about sister-brother relationship (I would still find that gross, but not sinister). Still, parent-child is an exceptional case.
Ftr, I don't think they should go to jail.