Go to the FAQ, the glossary and the archive about stress and the ones about skilled relaxation for THAT understanding. Besides SR, I should probably also go to Church and set my mind straight to think of less stressful activities. The sexual activity is changing your hormone balance which, in a person with adequate stress reserves, would be perfectly normal and cause no acne. Hopwever, if you are close to your stress-effect limits, sexual activity changes can be enough to further unbalance your hormones. Hope this is making sense.
Prayer is another way to do the same thing and many people do it just that way.