Endorphins that are released during sexual arousal have a peripherally vasodilating effect. This means that not only will the blood rush to your penis (or her clitoris/nipples/other sexual organs), it will also rush to other parts of the body, like the surface of the skin. Because blood contains heat generated by the body, lots of blood in an area tends to make that area feel hot -- think like, a swelling from an injury. The injury swells -- lots of blood -- lots of heat.
Endorphins released from sexual arousal also have many other purposes -- they make some glands release fluids (a girl getting 'wet', 'precome' for a guy,) causing salivation, perhaps eye-watering, etc. Muscle tension generally decreases. Have a look into your partner's eyes next time you're in the middle of the 'act' -- I bet their pupils will be wide as SAUCERS.
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