Iï¿½m going to start this out with some shocking news: sex is good for you. (Of course, it can also get you in a lot of trouble, but thatï¿½s another story.) Why? There are many reasons, but chief among them is undoutedly the fact that researchers have uncovered the fact that sex is actually an excellent way to boost testosterone. Thatï¿½s right ï¿½ itï¿½s time to party!
For example, one 1992 study examined four couples on days that they did and didn't have sexual activity. As you can probably guess, they found that testosterone levels increased on nights after sexual activity and did not on nights where there was no intercourse.  The authors noted that their study showed that it was likely sex that increased testosterone and not the other way around.
This was verified in a study of senior men that found that more sex corresponded to higher testosterone (although there is a chicken-and-egg issue here).  Regardless, studies show that just the anticipation of sex in animals will increase testosterone levels.  Other researchers clearly state that "lack of sexual activity due to erectile dysfunction (ED) decreases testosterone (T) levels through a central effect on the hypothalamic-pituitary axis." 
It is interesting because as erectile dysfunction sets in, many guys believe they have overdone this area of their life and fall prey to the strange underworld of "overmasturbation" sites and such. However, barring extremes, it is actually lack of sexual activity that can cause middle aged males problems. You can go too far, of course, and aggravate the prostate and pelvic floor muscles, so some maturity and moderation is always warranted.
In fact, this was verified in the same study above  by giving males either Viagra or Cialis and the Cialis group had significantly higher testosterone levels than the Viagra group. Of course, both of these drugs are PDE5 inhibitors and, therefore, boost nitric oxide, allowing more blood to flow into the penis. However, Cialis has a significantly longer half life. The rule of thumb is that Cialis is effective for about 36 hours for most guys and Viagra for one evening. The researchers noted that sexual intercourse frequency was significantly higher in the Cialis group. This means that the higher testosterone levels of the Cialis cohort was likely because they had more confidence and better erections on most nights than the Viagra group. The researchers concluded that "as it is unlikely that the two drugs have a different direct effect on the pituitary-testis axis, this effect is probably due to the higher frequency of full sexual intercourse."
NOTE: Read my link on How and Why Sex is Good For You for more reasons as well.
So the bottom line is that multiple lines of evidence point to the fact that sex actually increases testosterone through mechanisms that are not thoroughly understood and should be considered one of the engines of increasing oneï¿½s androgens naturally.
However, that is just half of the story. Increasing testosterone can significantly improve your sex life and lowering it can do the opposite. Testosterone could be called The Sex Hormone for good reason: it is critical to literally every step in the love making process and can even play a role in relationships, your physical appearance and many other important bedroom factors.
Below are 10 Critical Testosterone-Related Sex Life Factors where testosterone has a strong and significant impact. Many men struggling in the bedroom are struggling because they have low testosterone. When you read the list below, you will see exactly why:
1. Sexual Desire. Many studies over the years have shown that testosterone is critical to sexual desire.  One of the most common and revealing Symptoms of Low Testosterone Males is a loss of libido. Many males write in saying that at one time in their life they had trouble not thinking about sex and now they almost never do. This can be very hard on relationships.ï¿½
2. Nocturnal Erections. Low testosterone does not just attack your sex life in the day but at night while you are sleeping as well: morning erections are very dependent on testosterone levels, especially ones in the hypogonadal range, and a number of studies have correlated low T levels with decreased "morning wood".  The significance of this is that the penis requires regular oxygenation (from blood) or the internal structures can be negatively impacted. For more information, see my link on The ABC's of Morning Erections.
3. NOS (Nitric Oxide Synthase). One thing that many doctors do not realize: testosterone levels directly impact NOS activity. Animal studies have shown that castration decreases eNOS's superpowers by about half  and testosterone restores it back to normal  Of course, the significance of eNOS is that it is the all-important enzyme that governs how much nitric oxide is in your arteries and blood vessels, including those in your penis.
4. Anxiety and Mood. If you've poked around this site much, you know that testosterone strongly affects the male brain. And low T can make you just plain hard to live with. A number of studies have shown what just about every hypogonadal guy describes: anxiety, nervousness, irritability, mental fog, etc.  It also can make you forgetful - always a favorite trait with women, eh?  Again, this makes relationships very difficult and puts yet another barrier in one's sex life.
5. Depression. A significant amount of research has shown that low testosterone can also cause or make worse existing depression.  Many physicians are now noting the very important role that hormones, especially testosterone, play in treating depression. Of course, depression makes relationships and sex very difficult.
6. Venous Leakage. What literally rots the inside of your penis? If you guessed "lack of testosterone," you'd be right on. If this goes on too long, it can result in a nasty condition called venous leakage, where as much blood leaks out of the penis as leaks in. For more information, read my link on Venous Leakage.
7. Hardening of the Arteries. Low testosterone often attacks every artery in your body, including those in your penis.  Several studies have shown that it raises risk for arteriosclerosis. And that's never good for your sex life. There are many reasons for this by the way, including the fact that low T increases weight gain, insulin and inflammation - a truly wicked combination.
8. Penile Muscle Atrophy. The muscles at the base of penis are particularly important for erections. And they are particularly vulnerable to drops in testosterone levels. For more information, see my link on The Importance of Kegel Exercise for Men.
9. Diabetes, Metabolic Syndrome and High Blood Pressure. These three bad boys are some of the biggest risk factors for erectile dysfunction (and many other medical conditions). And it may come as no surprise that low testosterone increases the risk for every one of them. For more information, see my book Low Testosterone By The Numbers.ï¿½
10. Sleep and Fatigue. Inadequate testosterone can also lead to poor sleep quality, which means that the only thing you'll want to do under the covers is put your head on the pillow and crash. Testosteorne deficiencies decrease the all-important recuperative REM sleep and other critical sleep parameters. One study showed that low T did this not only by increasing weight gain, which negatively impacts sleep in and of itself, but also through other mechanisms that we do not yet understand.  And none of this should be a surprise considering how much that testosterone affects the brain.
And, by the way, all of this underscores the fact that the PDE5 Inhibitors will not always solve a man's problems if his testosterone is low enough. A man is definitely swimming upstream without his hormones cooperating.
1) Physiology & Behavior, Jul 1992, 52(1):195-197, "Male and female salivary testosterone concentrations before and after sexual activity"
2) Science, Nov 28 1980, 210(4473):1039-1041, "Classical conditioning: induction of luteinizing hormone and testosterone secretion in anticipation of sexual activity"
3) Journal of Gerontology, 1982, 37(3):2880-293, "Relationship of Serum Testosterone to Sexual Activity in Healthy Elderly Men"
4) Clin Endocrinol (Oxf), 2004 Sep, 61(3):382-6. "Type V phosphodiesterase inhibitor treatments for erectile dysfunction increase testosterone levels"
5) The Journal of Clinical Endocrinology & Metabolism, Jul 1 2006, 91(7):2509-2513, "The Relationship between Libido and Testosterone Levels in Aging Men"
6) Fertil Steril, 1995 May, 63(5):1101-7, "The effect of androgen on nitric oxide synthase in the male reproductive tract of the rat"
7) Int J Impot Res, 1995 Dec, 7(4):209-19, "Nitric oxide mediated erectile activity is a testosterone dependent event: a rat erection model"
8) Journal of Andrology, 18(5):522-527, "Relationship between sleep-related erections and testosterone levels in men"
9) Asian Journal of Andrology, 2010, 12:136ï¿½151, "Androgens and male aging: current evidence of safety and efficacy"
10) ARCH GEN PSYCHIATRY, MAR 2008, 65(3):283-289, "Low Free Testosterone Concentration as a Potentially Treatable Cause of Depressive Symptoms in Older Men"]
11) J Clin Endocrinol Metab , 2000, 85(8):2839-2853, "Transdermal testosterone gel improves sexual function, mood, muscle strength, and body composition parameters in hypogonadal men"
12) Oregon Health & Science University. "Testosterone Deprivation Makes Men Forget."ScienceDaily, 22 Oct. 2004. Web. 23 May 2011
13) Nephrol Dial Transplant, 2011, "Low serum testosterone, arterial stiffness and mortality in male haemodialysis patients"
14) The Journal of Clinical Endocrinology & Metabolism, Feb 1 1997, 82(2):682-685, "Association between Plasma Total Testosterone and Cardiovascular Risk Factors in Healthy Adult Men: The Telecom Study"
15) J Clin Endocrinol Metab, 2008 July, 93(7):2602ï¿½2609, "The Association of Testosterone Levels with Overall Sleep Quality, Sleep Architecture, and Sleep-Disordered Breathing"