Human male sexuality refers to the sexual function and behaviour of men. Sexuality is a combination of both our biological evolution and environment (social / cultural construction). Views of male sexuality have changed over human history and through out different cultures.
Male sexuality encompasses a diverse range of issues, behaviours and processes e.g. male anatomy, genes, hormones, psychology, desire, gender, identity and masculinity, relationships, orientation, orgasm, masturbation, sexual intercourse, evolution, sperm competition and reproductive strategies, promiscuity, monogamy, morality etc.
Sexual health topics particular to men include prostate health, premature ejaculation, infertility and erectile dysfunction. "Disorders of sexual function are common among men of all ages, ethnicities, and cultural backgrounds. It has been recently estimated that more than 152 million men worldwide experienced erectile dysfunction in 1995, and that this number will rise by 170 million, to approximately 322 million by the year 2025." 
The development of oral medications (such as Sildenafil citrate, sold under the name Viagra, since 1998) to treat erectile dysfunction has raised awareness and been commercially successful, however lifestyle changes such as exercise and nutrition may prevent erectile function. 
The penis, and penis size in particular is a common way that men identify with being masculine. "The average unstretched flaccid penis length ranges from 8.85 cm to 10.7 cm, stretched flaccid length ranges from 12.45 cm to 16.74 cm, and erection length ranges from 12.89 cm to 15.5 cm. Normally, the testis increases in size from 1–3 cm during the neonatal period of life to 15–30 cm in adulthood. A normal size adult testis has dimensions of 4.1–5.2 cm in length and 2.5–3.3 cm in width." 
Some evolutionary theorists speculate that penis size is less to do with successful copulation, and more to do with sexual selection relating to dominant males. This is consistent with the fact that as we evolved to walk on two legs, our genitals became more exposed, thus providing a more obvious evolutionary mechanism (evolving a larger penis) as a sign of male dominance.
Evolution also suggests that violence is predominately a male phenomena in the animal kingdom. "When it comes to sperm makers, success is likely to crown those who out compete their rivals, and so, in species after species, it is the males who are larger, nastier, more likely to be armed with lethal weaponry and a violent disposition to match. Natural selection has outfitted males with the tools for success in male-male competition, much of it violent." 
Editors: Karene Jade Howie
1. Ayta IA, McKinlay JB, Krane RJ (1999) 'The likely worldwide increase in erectile dysfunction between 1995 and 2025 and some possible policy consequences'