The fact that a meme is being a meme aside,
I'd say its the 'intention' (or 'free will' if you will) of the wearer is the point:

Wear undergarment: Wearer does NOT want her figure to be noticed

Wear Bikini : Wearer does NOT MIND / WANTS her figure to be noticed
Saying they are the same is just stating the obvious without considering the social part of wearing clothes (which is inconsiderate)

Rather, saying bikinis and undergarments are the same,
is like saying that my wife and your daughter is the same, a girl;
then i can f*** your daughter cause she's the same as my wife, a girl... ...