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Before sharing some of the results of my research and some of the conclusions I've reached, I'd like to put two questions: Do people generally feel that all sexual contact between adults and significantly younger partners is "abusive"? And do the younger partners necessarily suffer psychological harm as a result of such contact? In short, apart from starting these threads in the first place, do I need my head examined?
1. No, but I should qualify that. A 40 year old and a five year old? Abusive. A 40yo and a 17yo? Not necessarily. Unconventional indeed, could be abusive depending, but not necessarily. A 21yo and a 12yo? There's a grey area. Probably not a healthy relationship.
I have a hard time shedding the trappings of a Mennonite history, it is admittedly hard for me to think of sexual contact without the context of emotional relationship. Back up, of course I can (and do

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think of it, but in reality, the one time I had sex where no relationship was involved was more disappointing than anything... So when I think of these things, I think of a relationship, and it is hard for me to imagine a healthy adult finding a satisfying emotional relationship with an adolescent.
Over all, for me there is a sort of line of demarkation, and that is puberty. Two six year olds may certainly be able to "play doctor," it's natural; and I feel decidedly less squeamish about the age difference when both parties are post-puberty.
2. No, they don't suffer psychological harm
necessarily. I fear it is, however, more likely than not. I don't trust that report you mention, at least not at first glance, because humans are so complex. How is psychological harm measured?
Growing up, I had a friend who was a very "sexual" person. He first received oral sex at age 8, from an older baby-sitter. This will sound like a Penthouse "Forum" letter, but oh well. He was my friend. He had, shall we say, certain oversized proportions. He led a very sexualized adolescence, and is now happily married, and seems normal in every way. I don't feel like he was harmed by the early start. He was always more libidinous than the rest of us. There are probably just some people like that. You may be one of them. I don't think most kids are.
Having written that, perhaps (I'm trying to be open minded) I should adjust my statements. Given there are people like that, they are seeking out consensual sexual experiences at a young age. Most kids are NOT seeking it, and it is usually visited upon them without choice in the matter, and THAT is abuse.