A. Don't like it, for the reasons that you explained.
B. I don't mind this at all. They're about the same age, and not blood related...
C. I don't really like this one either, I mean the parent would be so much older than the child, and the step father/mother has to look at themselves as the child's actual parent, and not someone who can just marry them... The child would view them as a parent (I'd think...) and it would just be too weird... The older one should be much wiser any way, and know that they shouldn't do something like that. Do I sound conservative?
(Dan beat me to the Woody Allen example... Ugh, the guy's a genius with great writing and film skills, but he's a pervert... Apparently, he took nude photos of his step daughter when she was 13... Ick.)