1.) You can be naked and be seen by others and not feel shame, or look on a naked person and not feel shock, disgust, or lust. It requires either being not raised with society’s neuroses about sex, clothing, and the human body - admittedly rather difficult to avoid entirely if you went to school - or unlearning it. Even then for most of Western History people weren’t all that prudish compared to what they became starting around 1750 or so (and it was actually more much severe than the present day around 1860 to 1930).
2.) All that stuff about “sin” seems like a bizarre argument from a modern perspective, but there’s a way it mostly makes sense. Between the fall and the Resurrection, there were supposedly two forms of “sin” - disobedience to God, and impurity. Impurity builds up over time, as a result of the Curse of Original Sin™, but could also build up faster due to engaging in “impure” acts. Only by engaging in “purity” rituals, various animal sacrifice ones being common, could it be removed. (Disobedience is another matter.)
After the Resurrection, supposedly one could avoid needing a lifetime of purity rituals by accepting Jesus as one’s “Lord and Savior” (originally, in a single ritual witnessed by other Christians) but it wasn’t necessarily required. It wasn’t until later that the Jewish rituals were universally considered retroactively invalid post-resurrection, and accepting Jesus was the only way… thus humans could no longer save themselves from God’s punishment, at least not for the impurity stuff. There’s still multiple levels of WTFery involved in this, but getting into any of it would make this post waaayy too long.
3.) I’ve spoken to Christians about whether nudity was actually inherently sinful. The answers were all over the place, ranging from nonsense like Ken Ham’s, to “No, thanks to humanity’s new Covenant with God post-resurrection” to various forms of “mostly yes, but also sort of no” to even admitting to having no idea. One guy even said that no, not inherently, but you can’t truly earn Heaven unless you’re tempted to sin but resist temptation, so it’s in society’s best interest to sexualize nudity. So no agreement to this even among Christians.