If I understand correctly, the anti-contact view begins with the idea that adult-child sex is wrong. What's the reason behind that belief?

My fundamental position on the issue is one of "recusal". The world is full of adults who deeply care about the welfare of children. Pedophiles seem to have a vested interest in deciding adult-child sex is OK, we have no special knowledge about the nature of children, so we should defer to those who don't seem to have that vested interest.

Going beyond that to my personal judgment... I think there is far more danger of harm in allowing adult-child sex than in prohibiting it. The potential for abuse of power, misreading of a girl's feelings, or the girl's later regretting what she did are great -- and society is full of women who feel damaged by their early sexual experiences. The cost to prohibition is a few girls who would genuinely like to have sex with adults who are unable to do so. The silence among adult women on regret along those lines is deafening. The prohibition also provides a defense of underage children from genuine, indisputable rape. Given the "beyond a reasonable doubt" standard, a man raping an adult woman can claim she consented at the time but is lying about it now and often win. Prohibiting adult-child sex removes that defense. Legalizing it opens the door to much rape by unscrupulous men.