Sex is good, and these are all posts...
Hormones are released and as with anything, our skin goes through a perpetual chemical balancing act. However, sometimes i think sex (the act) isn't what is causing it as opposed to the actions following.
Whether you work hard enough to actually sweat (eh hem) or not, the pores are opening because you are raising your heart rate and body temperature. Hormones are being released and sweat glands are getting ready for a workout.
When you are done, I know we are all cuddling (or passing out) and not all jumping up to wash our faces, necks and behind our ears, right! It is a good idea to wash hands, face, etc after sex and then returning to bed. It will help keep the acne from appearing the morning!