No doubt a powerful and controversial subject, sex has many different meanings, results, benefits and risks.

Sex as Exercise
Sexual activity can burn 150 calories in a half-hour, so you could view it as a form of exercise. All the same benefits of spending time in the gym, including cardio exercise and improved circulation, lower cholesterol, elevated good cholesterol, flexibility, isometric and dynamic muscular exercise, and the release of feel-good endorphins.

Sex as an Immune Booster
Dr. Carl Charnetski, professor of psychology at Wilkes University in Wilkes-Barre, Pa studied 112 college students, who reported the frequency of their sexual encounters and were divided into four categories: none, infrequent (less than once a week), frequent (one to two times per week), and very frequent (three or more times per week).  Individuals in the frequent group showed significantly higher levels of IgA than the other three groups.

Sex as a Mood Booster
People who have regular sex know that they handle stress better, feel relaxed and sleep better. A study by Stuart Brody of the University of Paisley in Scotland found that penile-vaginal intercourse, but not other sexual activity was associated with better psychological and physiological function. People who had penile-vaginal intercourse had lower baseline BP and lower BP reactivity to stress of public speaking and verbal arithmetic compared to people who had sexual activity without penile-vaginal intercourse or people who had no sexual activity.

Sex and Longevity
Research by Smith and others at the University of Bristol studied 918 men aged 45-59 in the town of Caerphilly, South Wales and five adjacent villages. All deaths and deaths from coronary heart disease were compared to orgasmic frequency. Mortality risk was 50% lower in the group with high orgasmic frequency compared to the group with low orgasmic frequency. Deaths were most marked for coronary artery disease-related deaths.

Brody S.  Blood pressure reactivity to stress is better for people who recently had penile-vaginal intercourse than for people who had other or no sexual activity.
Biol Psychol. 2006 Feb;71(2):214-22. Epub 2005 Jun 14

Charnetski CJ, Brennan FX.  Sexual frequency and salivary immunoglobulin A (IgA).
Psychol Rep. 2004 Jun;94(3 Pt 1):839-44.

Davey Smith G, Frankel S, Yarnell J.  Sex and death: are they related? Findings from the Caerphilly Cohort Study. BMJ. 1997 Dec 20-27;315(7123):1641-4.

Gallup GG Jr, Burch RL, Platek SM.  Does semen have antidepressant properties?
Arch Sex Behav. 2002 Jun;31(3):289-93.