"On what grounds is it appropriate to claim that iatrogenic factors are the primary cause of harm from sexual abuse, while not appropriate to claim that when people speak of "consent" most of them mean "informed consent"?"

It is clear that iatrogenic and sociogenic factors are large causes of harm even in cases of genuine sexual abuse (witness the difference in outcomes for rape victims from cultures where extramarital sex is widely condemned as opposed to cultures where extramarital sex is broadly accepted). In cases where abuse is disputed iatrogenic and sociogenic factors are clearly even greater causes of harm, and it seems fair to say it is the primary cause of harm - though of course it is difficult if not impossible to make empirical claims about emotional harm. I wonder why you did not distinguish actual from imagined abuse, as we usually do here.

As for why we should not presume that most people mean "informed consent" when they say "consent" - there are two good reasons. First, most people haven't even thought about the distinction, and second, no one of any age can foresee all future consequences of their actions - making informed consent impossible for anyone who has not already lived forever.

Incidentally, in light of the fact that informed consent is impossible, I have decided lately that I will support the age of consent being raised to "eternal less a day". Any person who is less than an eternity old should not be held criminally liable for mutually agreeable sexual acts because they do not have the maturity to understand the full consequences of their actions. Of course, laws against rape will still stand, as most people will understand the immediate and negative consequences of their actions.