A fuck buddy is a person with whom one may have "no strings" sex on demand, thereby giving the people involved an outlet for their sexual urges without the potential stress and time-demands of a committed relationship. Two people may elect to become fuck buddies because they are unwilling to commit to a full-fledged relationship for whatever reason. The idea is necessarily hedonistic.
The concept of the fuck buddy is an increasingly prevalent social phenomenon, particularly among young women aged 18 to 35 who understand Contraception. It has been popularized in recent years by the US sitcom Sex and the City.
This arrangement is particularly suitable for young professionals, who put a lot of their free time into their work and therefore don't have time for a "proper" boyfriend or girlfriend. It also allows people who have excellent sexual chemistry to enjoy each other, even if they don't click on other levels. And that's not even getting started about the orgasms.
The major cause for the breakdown of fuck buddy-type relationships is when one party starts to develop a romantic attachment to the other. In the instance where both parties start to feel this way, a traditional relationship may develop. However, when these feelings are unrequited, it can lead to jealousy, feelings of betrayal or feelings of being used.
The concept of a fuck buddy is not in accordance with traditional moral values, particularly religious values, and is generally frowned upon by the traditionally more conservative older generation. It always happened but had different names. However, the fuck buddy term and idea has yet to penetrate very deeply into the public consciousness and so the kind of large scale backlash by religious groups that one might expect has yet to appear.