The word "subtil" is the english translation of the hebrew word " `aruwm " which comes from the root word " `aram " which means to to be cunning (usually in a bad sense):--X very, beware, take crafty (counsel), be prudent, deal subtilly.
Interestingly, the word "naked" is the english translation of the hebrew word " `eyrom " which also comes from the root word " `aram ".
Nakedness is evil, for the sight of it leads to carnal desire and carnal desire to sexual activity for its own sake. God created the sexes, male and female, primarily for the pupose of procreation (Gen 1:28 "...Be fruitful, and multiply....." The pleasures of sexual activity are intended as a bonus, an added incentive only. Once their eyes were opened, by the eating of the fruit, they realized that they had been sinning, for they had been having sex with each other only to satisfy their own sefish desires, only for the pleasure of it, and not for the pupose for which God had designed. They were ashamed of their sin and clothed themselves, albeit clumsily, to conceal what they now knew to be the ultimate source of that sin: carnal desire. On top of all that, they knew that they had sinned even further by disobeying the one and only commandment God had given them. No longer innocent, they became crafty, cunning, prudent, and subtle; like the serpent.