No, this is completely normal. Guy’s nipples can become erect (hard) during sexual intercourse. This is thought to happen in both males and females during sexual arousal due to the release of the hormone oxytocin (the same hormone that causes a woman’s nipples to become erect during breastfeeding).

Some guys enjoy nipple stimulation, while others don’t. According to a study on nipple/breast stimulation and sexual arousal in young men and women (published in The Journal of Sexual Medicine), nipple/breast stimulation causes or enhances sexual arousal in approximately 52% of young men.