Underwear
Male underwear is worn under clothes so as to protect the genitals from friction, or to provide support. While undershirts and long johns are also underwear, this article deals solely with below-the-waist underwear.
History
While loincloths have been worn since ancient times, underwear as we know it today has come into existence as a result of the invention of knitting machines around 1800.
According to Wikipedia, underwear first appeared in an advertisement in 1910. The jockstrap, which antedates the brief, first appeared in 1874 to provide support for riders of the newly-invented bicycle.
Until the 1970’s, male underwear was utilitarian and white.
Boys in underwear
Commerce in used underwear
Wearing panties
Panties (knickers in England) are by definition female underwear. However, men also wear them for fetishistic reasons, or sometimes for comfort. The practice is growing and becoming increasingly open and unashamed since 2000, at least in Western countries. Stores that sell women's underwear are increasingly comfortable with male patrons. While some women are uncomfortable with it, others think it's a hoot (fun), or sexually stimulating for them as well. While there is a well-developed community of male pantywearers (see http://www.hewearspanties.net), and occasionally there will be a picture of a leatherman with panties, as something super-kinky, the gay male community remains in general still devoted to the hypermasculine. Many more straight men than gay men wear panties.
Boys have started wearing panties too. In some cases these are gender-variant boys, who feel somewhat, or completely, feminine, and wear dresses and other female clothing as often and as publicly as they can. Other boys wear panties for the same reasons men do: they've seen pictures of men in panties on the Internet (typically masturbating) and find it a "hot", sexy thing to do. Some parents are comfortable with it - the dad may wear them too.