Because when your bodies are close together they produce heat and then it makes you sweat
We sweat when we need to cool ourselves down. Sex requires exertion which in turn creates heat and from two bodies...well if it's good it gets hot quick.
Because you put in a lot of hard work lol
Because, pretty much weather you want to look at it this way or not your exercising your body and your muscles
Almost any strenuous and time consuming activity involving the body will make it sweat. Sweat is used by the body to cool it down after any activity that has produced body heat.
Well people sweat during sex but most people don't notice the sweat until afterwards.....just like running, once you stop you can feel how hot and sweaty you are