I would say clothing is a tool if you think about all the things it is worn for. (ie. sex, title, warmth....)
Also, okay, so some cultures wear little clothing but they still cover up parts of themselves. I haven't seen a single tribe that hadn't been wearing at least some clothing in some form. Yes they may be bare-breasted but the women still wear loincloths.
And other animals aren't modest because they haven't developed that complex of emotions. They may feel pain, happiness, sadness and fear. But that's all basic.