I see many times that porn actors, who are fit and in shape, sweat during sex. they aren't sweaty at the beginning of the scene though.
is sex really that much tiring that it will cause the actor to sweat? even considering the actual length of the scene before editing?
These are just guesses:
The lighting required for filming might produce a lot of heat.
The thermostat might have been turned up because chilly temperatures can have an adverse effect on erections.
The actors might have been misted with water to make it appear that their exertions are more vigorous than they are.
They might actually have worked up a sweat.