@That3rdGuy: You didn't understand akari_mizunashi's comment. I'll try to explain without being too explicit.
"Shriveled genitalia" is the default for humans of any age. Males have an obvious sign when their genitalia is no longer shriveled. Females are similar though relatively subtle.
Sex can occur with two shriveled partners. Pregnancy has to have an un-shriveled male (outside of IVF and the like). The phrase "girls mature earlier than boys" partly refers to when they can maintain an un-shriveled state.
If the girl remains shriveled during the sex act, then the act is usually painful (hence groans instead of moans when the male isn't attentive). Yes, elementary school age girls have become pregnant. But the males in these cases are nearly universally older. Often times, they are 10+ years older and the females did not consent (and I don't mean in the statutory sense).
Long story short: for the humans, most conclude that "the brain is the biggest sex organ". 11 yr old x 11 yr old is not only biologically unlikely to result in pregnancy, but neither participant is likely to have the same mental state before and after the sex act regardless of pregnancy. Their dialogue was like two kids playing doctor or playing "I'll show you mine if you show me yours". Hence they "stick their shriveled genitalia together".
Sex education is not about teaching kids what to do. It is absolutely true that kids can figure that out themselves. But those kids tend to be ignorant not innocent.
And four chapters is more than enough to decide to stop reading a series. I have limited time and limited budget. I would rather spend my money elsewhere because of the above. At some point, a manga will explore pre-teen pregnancy (or even pre-teen sex) in a thoughtful manner but I don't feel that this is that manga.