One of the things I cover extensively is how Exercise Increases Nitric Oxide and Erectile Strength and Weight Training Increases Testosterone. In other words, almost any exercise is great for your sex life. However, there is one type that may be best of all: core training.
There are many reasons for this, but stop and think about what muscles you actually use most in the bedroom. The answer is clearly those right around your midsection: the abs, gluts and pelvic muscles. And, for those who don't know, core training is by definition training of those very muscles. Core training also gives you better posture, more definition in the ab area and a butt that's not flat, all of which are attractive to women the world over. (See my link on Females and Looks for the importance of your appearance.)
The importance of core training goes way beyond all of this, however. Core training helps males internally, especially in the areas of 1) oxygenating the penis and 2) building the pelvic floor muscles. These are especially critical for guys that are middle aged and beyond and males of any age experiencing a loss of erectile strength.
The pioneer in this area is Frank Sommer, a physician and researcher in the area of erectile strength, sports medicine and urology. He has many interesting papers, including ones that show 1) regular use of PDE5 inhibitors can be partially curative of ED, 2) improved erectile function in men taking Viagra daily for a year and 3) a study of cycling and erectile dysfunction.
As interesting as all these are, perhaps his most significant contribution is studies on the effect of core training on erectile function. What he found is that core training is the best exercise to actually oxygenate the penis and strengthen those all-critical pelvic floor muscles.
Oxygenation of the penis is absolutely criticial in order to keep the tissues within the penis that trap blood for an erection flexible. If these tissue become "hardened and inflexible", then it becomes increasingly difficult for a male to keep blood trapped in the penis. This isn't an issue for the young guys, but middle-aged and beyond males can almost always use some help in this area, especially if their morning erection count and duration begin to decrease.
Pelvic floor muscle strength is critical in a similar way. The muscles at the base of the penis are activated upon arousal and tighten trapping blood in the penis. Of course, Kegel Exercises help in this area, but core training is likely just as good. If these pelvic floor muscles weaken or atrophy due to disuse (or low testosterone), then exercising them becomes critical in order to achieve and maintain an erection.
Frank Sommer actually studied which core training exercises led to the greatest oxygenation and muscle strengthening and put it in a set of books called Vigorrobic: Increased Potency Through Specific Fitness Training. For most guys the difference that these exercises can make to their sex life is very significant. Sure, running a couple of miles is great, but when it comes to bedroom performance: go for the core!