Seeing sites for married people to have public extramarital affairs and even consultancy sites that encourage such relationships are painful on my eyes as it caused such a bulging!
Googling [dating married people], I am informed that there is a total of 6.16 million results that could relate to the keywords I had given and the first page of results contained 8 out of 10 that are pro-extramarital relationships. Then I Blackled [dating married people] and the first page of 10 results presented 9 sites that are pro-extramarital affairs. Existence of so many extramarital affair sites suggests that such relationships are quite omnipresent. The question now is "Why do people date married people?"
For one, it is not as though the person actually goes around targeting married people to date, but usually the case is that they just so happened to meet or know each other from a previous time, say few years back or childhood. From this meeting, and sometimes from another one or two meetings, a bond could develop or a situation of possible companionship occurs. When exactly the information of a person still being married to another is given or how it is communicated it will not be factored into this article for the purpose of ease of discussion. Beside, the fact that the affair is ongoing eliminates the need to look into that detail.
Back to the question at hand, I believe the following are contributing factors to why a person, single, married, divorced or widowed, would date a married person.
1. A person thinks or dreams that the married person will leave his / her partner for him / her. If this has been communicated to the person by the married individual that thought or dream would be further strengthened. Why that person would hope on such a notice could be linked to egotistical matters. The thought of being chosen over someone else is always an ego booster. In cases where the person is going through emotional instability (eg recently dumped / divorced / widowed) or is naturally introverted, he / she clings on to that hope even more determinedly.
Reason I am going with the ego-booster is because of the persistency of chasing a married person despite there being billions of people around the world amongst which many more "appropriately" qualified suitors could be found.
2. The risks involved provide an excitation or interesting factor to one's life. This is usually the case for those who are experiencing mundane or repetitive daily lives, or for adrenaline (sexually) junkies. Risks included include being approached by the married person's current partner, family / friends / general public find out about the situation, or reputation / credibility at work or to the public being marred.
3. A person has the under belief of "One True Love". As corny as the idea sounds, such thinking does exist. Especially to a person who sees their surrounding environment as a constant negative or wicked world, that person would want to hold on to concepts such as fairy-tales and happy-ever-afters. Labeling someone as their Prince Charming / Princess Jasmine or One True Love who will come to rescue him / her from the "evil" surroundings and whisk them away to happiness gives a glimmer of hope that takes away their focus on negativities.
4. Some people get a kick out of having something that, or in this case, someone whom, they rightfully can not have. It is the competitive spirit to take something or someone away from another person.
5. A person feels kindly towards the married person's partner, and simply dates the married person to hurt the current partner. In this case, it is uncertain if the person has actual love or caring feelings for the married person.
6. Also an ego-booster, in situations where the married person is going through through marital relationship problems, the person feels like a savior for making the married person happy or happier. In this situation, the person envisions him- / herself as Prince Charming / Princess Jasmine.
There could be other undercoming reasons, but these are viewed as main and popular reasons. In summary, with so many motives it is no wonder that there are many people who would date a married person. As mentioned earlier, there are billions of people on earth, so it does not take a mathematically inclined person to estimate how big a subset of people dating married people there could be. One could argue on ethics or religious controlling the numbers, but human beings, though given a brain, are quite complex animals with difficult to control needs and wants at times.