Here's a theory as to why: the experience may indeed be painful in the psychosocial context of our present society, but perhaps only in that context, or more specifically, because of that context.
That is, we have ideas of shame—that certain things are, or are not shameful—that are culturally based, and when we do things that offend our (learned) sense of shame, we feel, and remember, the associated negative emotions, without necessarily remembering their cause. We associate the negative emotions with the circumstance, instead of the long-gone prior that caused us to feel shameful in such circumstances. In some religions, you can feel shameful working on the Sabbath; in our society, you feel shameful having sex when society says you aren't "ready" to. (I admit that that's a bit of a stretched analogy.)
The more common reply to your argument, though, is that the children are reassigning a negative emotional weight to their memory of the experiences, after the fact, because the therapist/parent/whomever is expecting the experience to be negative. They don't have to prompt for this verbally; they may be using completely neutral language, or simply asking "what happened?" Either way, their body language will show their emotional reaction to every word (and if a horse can do math based on our observed body language, we're obviously not very good at concealing it.)
To demonstrate my meaning: If one of my friends punched me in the arm, I'd interpret that as playful at the time. If a stranger did it, I'd interpret it as hurtful. I literally feel more pain in the latter case, because of this expectation. Now, if, some time later that day, my friend insulted my race, or some other category to which I belong that implied that he just wasn't my friend any more, I'd re-think that punch. I'd remember it hurting more.
Child abuse recountings are extreme versions of this. If you demonize the adult in the child's mind, everything they do is going to take on a negative connotation. They're going to start looking for the negative angle: a hug was really a rough squeeze; a toussle of the hair was really a hair-pulling, and so on. In this light, of course sex was a bad experience—it's extremely physical with all sorts of pleasurable/painful connotations which can be switched around or played with to no end (for example, BDSM is simply a shared agreement on a set of altered connotations.)
Let's see... My original question was, "if the children said they are harmed then why don't you believe them?" Your answer sounds very much like it isn't that you don't believe them, but that the harm is discounted because it's society's fault.
Yet the original question posed was whether children are harmed or not, not whose fault it was.
Suppose that all the harm (all the "psychosocial" bad feelings) is an artifact of society, rather than society's way of preventing the bad feelings that are a natural result of sexual abuse. What then? ...
What do you believe that most people on this site don't?
I'm especially looking for things that you wouldn't even mention if someone wasn't explicitly asking for them. Stuff you're not even comfortable writing under your own name. Making a one-shot account here is very easy, go ahead and do that if you don't want to tarnish your image.
I think a big problem with a "community" dedicated to being less wrong is that it will make people more concerned about APPEARING less wrong. The biggest part of my intellectual journey so far has been the acquisition of new and startling knowledge, and that knowledge doesn't seem likely to turn up here in the conditions that currently exist.
So please, tell me the crazy things you're otherwise afraid to say. I want to know them, because they might be true.