I think you are confusing the ease to declare something immoral with objectivity/inherence. It is easy to find reasons to subjectively declare sex with a 5 year old as immoral. As the age increases there comes a point where still holding that sex with that person is immoral becomes much more difficult.
Bingo. I think the confusion here has to do with morality being
subjective versus morality being
arbitrary. The two are not the same. All morality is indeed subjective, in that it is created by humans, based on their perceptions of reality. But that doesn't make it arbitrary.
An example of arbitrary morality would be if society suddenly decided that the color blue was inherently evil, and hence the democratic state passes laws to criminally sanction those who wear blue clothing. This is morality
without reason. Contrast this with the prohibition of having sex with 5 year olds. Since a 5 year old cannot consent to sex in a meaningful way, the act of having sex with them is both trespassory and likely traumatizing. And most importantly - if it's OK for you to rape/kill/harm another, then it's OK for others to do the same to you, i.e. what we call the golden rule. This logical principle undergirds the development of morality based on consensus, meaning that we agree about which things are moral and immoral based on
shared reasoning. Because morality remains subjective, certain individuals may reject this reasoning, for reasons either logical or illogical, but the overall consensus remains because the shared reasoning remains.
That said, some morality by consensus is arbitrary because the shared reasoning is weak or non-existent. Particularly traditional sexual morality - rules which limit or restrict sexual conduct between consenting adults. A perfect example is homosexuality, the moral prohibition of which is arbitrary in the sense that it isn't backed by anything resembling a coherent rationale. The "reason" is either an "ick" factor - a purely visceral and emotional reaction - or the fact that someone wrote 2000 years ago in what is likely a work of fiction that when a man lies with a man, it's an abomination.
Because morality is subjective, a pedophile can argue that sex with young children is OK in his world view, simply because he wants to be able to gratify his desire without social or legal sanction. But his reasoning is poor, so he isn't likely to gain any traction making the argument. I think why Realibad erred here in referring to the immorality of child molesting as "inherent" is that he implicitly understands that the reasons for its prohibition are good, so good in fact that he incorrectly concludes that the rule itself is inherent in reality, as if to say that either there is an external source of morality, i.e. "God" or else that mother nature actually cares about human behavior. One need not make such assumptions, however. All one must do is explain
why something should be considered wrong and hence prohibited.
So morality itself is subjective because ultimately it is man made and every person will decide on his or her own whether they agree with each rule. But that doesn't make it unmoored from reality because the rationale behind such rules can be very connected to reality. The trauma caused by rape is real. The fact that if it's OK for you to kill someone, that someone may just come and kill you, is very real. That's why the subjective nature of morality, though often problematic, isn't as problematic as is often supposed.