What is the problem with thongs?

i just saw a question asking if it was okay for a 15 or 16 year old girl to wear a thong and i thought of course. it's underwear and nobody except them is going to see it. some people said only promiscuous girls wear thongs and i don't understand how wearing a thong equals to being a slut. ive been wearing them since i was 12 and i'm 14 now and i am NOT a slut. i'm a virgin actually and i don't plan on having until i find the man i am going to marry. why do people have a problem with teenage girls wearing thongs?