Why do nipples become erect during sex?

Oxytocin release is responsible for the nipple becoming erect with excitement and stimulation, caused by contraction of smooth muscle underneath the skin of the areola which pulls on the overlying skin creating a goosebump-like effect. As the breasts continue to receive stimulation, oxytocin continues to be released into the bloodstream, which has it's most intense effects within the first five minutes after release, but can continue to increase contractions for up to an hour. But as breast stimulation continues, so does release of oxytocin. What this translates into is more intense uterine orgasms as well as the feeling of having your whole body involved in your orgasm. Also, about 1% of women can experience an orgasm just from having their breasts and nipples stimulated, which is called the oxytocin pathway to orgasm. Oxytocin is also responsible for that tingly feeling after sexual arousal and/or orgasm, as well as a sense of sexual satisfaction.