Why do your legs hurt after sexual activity?

It depends on the location of the discomfort.

If it's muscle, that's just a normal part of the muscle fatigue.

If it's an ache in the joints or radiating from the joints it's caused by inflammation of the joints, possibly even early stages of Arthritis. Try a couple anti-inflammatory pain relievers BEFORE exercise, such as aspirin, ibuprofen or Aleve (naproxin). The anti-inflammatory pain relievers don't just dull the pain, they also reduce the inflammation and minimize joint damage. A buildup of lactic acid causes the aching sensation.