I'm going to use "consent" and "informed consent" somewhat interchangeable, as do most people making claims about inability to consent do.

One argument is basically a "adults can overpower children so children cannot consent". Of course anyone reading this story would agree that such is a nonsensical reason. I remember reading a story that basically said all women are afraid of men because they are all potentially rapists because they can all overpower them (http://jpegy.com/epic/homophobia-the-fear-that-another-man-will-treat-you-like-you-treat-women-18270). I think 99% of people can agree that women and men can have sex, even if the man is (perceived as being) physically stronger and able to rape the woman. There is the small portion of the population who things that basically anything but lesbian sex is rape and even most lesbian sex is probably rape too...


Another is "children are relatively dumb compared to what they will be once they are adults". Which really is not that bad of a reason, unlike the first. This is comparable to saying drugged people cannot consent because they'll regret it later and its taking advantage of them being in a state of decreased cognitive abilities and they will later regret it. Just like giving a drunk person a high-five is physical assault- they cannot consent to a mutual hitting of hands so you really just slapped their hand against their will. And parents giving their children hugs and kisses is sexual harassment/assault.

Then there are the parent sovereignty people who think the child is their property and only they have the right to give consent for their children.

And last but not least are the " I'm certainly not going to defend letting children have sex with adults when there is a pedophile witch hunt going on so I'm just going to pretend to go along with it so I'll say its just wrong".

I don't think I ever really supported the idea myself- I remember someone I had just became friends with basically ask me if I had a problem with him being with a 12 year old when he was 16- a high school student with an ~elementary-aged girl and I remember saying I had no problem with it (although seeing as it was a long-distance relationship, maybe it was only because I knew sex wouldn't be happening). This was years before I realized I was a pedo myself. When I first noticed myself looking at a little girl I was on a high school trip and was thinking "need to not stare so other's don't notice".