Uhm. Boobies? Yes Chuck. I am sticking with boobies.

Honestly. I don't know. I think because it's more acceptable to walk around in a bikini because it's under the classification of a bathing suit.

Underwear has it's own classification as being something hidden and unviewable except by a privilaged few.

So supposing you have a really cute bra and undies set that you like. I'm sure you could tell everyone it was a bikini and they'd believe you.

I've seen some bikinis that I thought were underwear and vice versa.

So I believe it really depends on the person wearing said item and the general reaction for the public around you.