Nipples become erect (hard) because the muscles inside them and underneath them contract. This is thought to happen in both males and females during sexual arousal due to the release of the hormone oxytocin. Oxytocin is the same hormone that causes a woman’s nipples to become erect during breastfeeding. It’s not known exactly why nipples become erect during sexual arousal, but it may serve the purpose of signaling sexual arousal to one’s partner.
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