This is a true story that took place about 3 years ago.
One Saturday, my roommate and her boyfriend were out on a date. I was in the living room watching a movie when they got back in. They sat down and joined me. Seeing them on the couch flirting with each other, knowing they were going to get frisky later, in one sense made me jealous and in another turned me one a bit.
My roommate excused herself to the bathroom. We kept watching the movie without talking. I glanced over to catch her staring at me chest. First, I was a bit annoyed, but then I looked down myself and saw my nipples poking through. I had not been wearing a bra sitting around and getting a bit turned on had made them hard. I blushed and looked back at him with a brief smile before turning away. My roommate then returned, taking him away to her room.
On this evening, they were getting it on and got particularly loud. I turned down my TV volume so I could listen in as they each moaned in pleasure. This made me extraordinarily horny.