ask them why the penis and vagina with all their fitting [-with-each-other] parts and sensitive parts are fully developed at birth. Boobs are not. Adam's apples are not. Pubes are not. Eyes, brains, guts, legs, are not. Penises and vaginas are.

I don't think you want to go in that direction if you care to use "nature" to argue that man-girl sexuality should be accepted. Vaginas are decidedly not from birth meant to accommodate adult penises.

Making vaginas that way would have little metabolic cost. The actual state of affairs would suggest that in the environment of evolutionary adaptation, prepubescent girls hardly ever got an edge from being protected by men whose interest in them was primarily sexual (and full intercourse would make a man more likely to offer protection). Knowing the age at which the vagina is fully grown in relation to other aspects of puberty would be of interest to me, as it would suggest the lowest age at which girls typically had sex -- with even say a 1% probability. I've never looked into it.

But I don't see any of this as relevant to the question in today's world, which is something to decide based on modern morality and enhanced expectations for girls' lives. I'm not citing it as an anti-contact argument, I'm just saying it doesn't work at all as a pro-contact argument. Pro-youth sexuality, it might conceivably have some relevance.