argues that we are psychically deformed in the same way as the victim of cerebral palsy or thalidomide is physically deformed. The implication of that and your logic is that every user of a wheel chair is there voluntarily.

The reality is that this world is fallen from the good state that God created it as. Instead we are faced with a prevalence of dangerous things that have negative consequences for the users and those around them - of which illegal drugs are another example. Just because something 'feels good' doesn't mean that it won't do long term damage - which is the claim of those who regard adult child sex as a bad thing. It's a logically consistent position based on the stories that are out there of people whose lives have been messed up by such events. The fact that there are counter examples doesn't prove that it is ok - any more than the fact that some people can function effectively despite being heroin addicts proves that heroin is therefore ok.