There's absolutely no question that great, consensual sex feels amazing.
However, there are three principal physiological elements why someone feels sexual pleasure: the pudendal nerve, dopamine, and oxytocin.
This is a huge, tender nerve that allows someone's genitals to send signals to their brain. In people who have vaginas, it has branches in the clitoris, the anus, and the perineum. And in people who have penises, the pudendal nerve branches out to the anus, the perineum, and the penis. It’s, however, important for women to realize that the nerve doesn’t have much concentration inside the vaginal canal. Most of the pudendal nerve endings are focused on the clitoris. That's why it's common for people who have vulvas to struggle reaching orgasm from penetrative sex alone.
This is often called 'the love hormone. It’s what makes us feel attached to people or things. Oxytocin is released during sex and orgasm, but it's also released when someone gives birth to help them feel attached to their baby. That’s the big one that makes you feel like your partner is special and you can’t get enough of them.
Just like oxytocin, dopamine helps your brain make connections. It connects emotional pleasure to physical pleasure during sex. So this is the hormone that makes you think, let’s do it again and again and again.
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Oxytocin and dopamine are both in a class of hormones considered part of the brain's reward system. As someone's body reaches orgasm, they flood their system because the brain is essentially trying to medicate them. The brain seems to mistaken sexual arousal as trauma. As someone gets aroused, their heart rate increases, their body temperature goes up, and their muscles tense, all of which happens when someone's body is in trouble, too.
Desire is ruled by different hormones, most prominently testosterone. But even that might not fully explain why someone isn't interested in having sex. While everybody has a pudendal nerve and can experience the release of dopamine and oxytocin that happens with sex, not everyone will experience that release as pleasurable or experience the same level of pleasure. According to Dr. McGuire "People are very complicated".