Briefs
Briefs are among the most utilitarian of men's underwear and are also the most commonly worn. However, while they may be inexpensive and convenient, they don't carry the nickname of "tighty-whities" for nothing. Their restrictiveness can have a disastrous effect on your sexual health. For starters, they hold your testicles too close to your body, preventing them from cooling themselves adequately. These important glands require strict temperature regulation in order to function properly. When they become too warm and are unable to cool down, your sperm count decreases and testosterone levels drop significantly. Furthermore, the elastic around the groin area can hinder circulation, depriving the genitals of essential blood flow.
Reduced testosterone carries with it a number of unpleasant, and even dangerous, side effects. It causes physical strength and muscle mass to wither, gives rise to mood disorders, potentiates hair loss and promotes obesity. It also allows estrogen levels to rise above what is considered normal for men, which scientists now know to be a strong contributing factor in the development of prostate cancer. In addition, low testosterone is practically synonymous with low sperm count and poor motility.
The restriction of blood flow caused by the elastic bands around the groin can lead to diminished nutrient and oxygen delivery to the testicles, further hindering their proper function. Besides that, reduced blood flow to the genitals makes it much more likely that you'll suffer from erectile dysfunction and decreased sensitivity.
Boxers
If you want to ensure proper reproductive function and sperm health, then boxers are strongly recommended if you must wear underwear. Their lack of restrictiveness means that your testicles are able to better regulate their temperature by extending away from your body as needed. This allows the proper production of male sex hormones and also encourages stronger and more numerous sperm. In addition, the lack of constricting elastic around the groin ensures good circulation to the genitals, resulting in improved erection quality and sensitivity.