Boys stink, throw rocks at them.

Some men think monogamy is what furniture is made of.

Insights into a male brain (or the lack thereof).

Promiscuity is wired into a man's brain and is a legacy of his evoluntionary past. Throughout human history, wars greatly diminished the numbers of men so it made sense to add to the size of the tribe as often as possible. The number of men returning from battle was usually lower than at the start. This meant that there was always a large number of widows, so creating a harem for the returning males was an effective survival strategy for the tribe. This is the way it was for hundreds of years. In addition, modern man is still equipped with a large hypothalamus (centre in your brain for sex) and enormous amounts of testosterone to fulfil his ancient urge to procreate. The reality is that men, like most primates and other mammals, are not biologically inclined to complete monogamy (Sad bitter truth Part I).

It has to be understood however, that in discussing men's urge to be promiscuous, we are talking about biological inclinations. We are not providing men with excuse for infidelity.

e.g. The brain circuitry of a moth gives it an instinctive attraction towards bright lights and this allows the moth to navigate at night using the stars and moon. Unfortunately, the modern moth is also living in a world that is dramatically different to the one in which it evolved. We now have moth and mosquito zappers. By doing what is natural and instinctive, the modern moth flies into the zapper and is incinerated instantly.

In understanding their biological urges, modern man have a choice of avoiding self-incineration as a result of doing what comes naturally.

The Sad bitter truth (Part II).
A rooster is a very randy male bird which can copulate with hens almost incessantly, more than 60 times in a given mating. He cannot however, mate with the same hen more than five times in one day. By the sixth time, he completely loses interest and cant 'get it up' but, if presented with a new hen, he can mount her with the same enthusiasm he did with the first.
A ram will not mount the same ewe more than five times, but can continue to mount new ewes with tremendous zeal. Even when the ram's former sexual partnersare disguised with perfume or bags over their heads, the ram still cannot perform. You just can't fool them. This is natures's wary of ensuring that the male's seed is spread as widely as possible to ensure the survival of that species.
For healthy young men, that number is also around five. On a good day, he can have sex with the same woman five times but will usually fail to give a sixth encore. Introduce a new female however, and, like roosters and rams, his interest (along with parts of his anatomy) can ripidly rise.