"Kupu-kupu" or so-called "Butterfly" in English includes in the kind of insect. It has a slender body with knobbed antennae. It also has a pair of wings to fly itself. When flying, the wings portrayed as the most salient part, since the wings are colorful well as reflecting its beauty.
Consequently I think that "kupu-kupu" with its salient beauty is possibly related to a woman in general. It is believed that every woman has these three absolute things: sex appeal, attractive body, and beautiful face. These are the nature of becoming a woman. So this metaphor is considered the positive one. On the other hand in Bahasa, we also have this kind of metaphor "Kupu-Kupu Malam" and it is usually used to point at the negative one.
Sadly "Kupu-Kupu Malam" refers to a prostitute woman in which she willingly gives a man all of herself in return for amount of money. Typically this kind of woman is what a man needs when his partner is out of expectation. He needs a woman who can satisfy himself, in particular, his lust. Furthermore, a man also likes a woman who makes an effort to win his heart with her attention.