Mate poaching, when defined, shows a cold, almost detached description. American evolutionary psychologists at the University of Texas, David Buss and David Smitt first coined the term in 2001 in a research paper. Mate poaching is the act of attempting to (or successfully) steal a partner already in a relationship. 
Mate poachers have statistically been shown to resort to several forms of manipulation and lies to maintain their "hunt" for their target that is already in a relationship. Many are themselves in committed relationships and tend to concentrate affection for those partners severely during "poaching season". 
While many may attribute culture to be the determining factor of mate poaching, it turns out that the practice transcends political and geographical borders. A study was conducted across 53 different nations and 16, 954 participants to show these results. There is an important factor to note though, in areas with little financial or natural resources and access, mate poaching is a record low. The study also cited, "The occurrence of attempting a short-term mate poach was significantly higher for men than women across all regions" but also noted that especially in western cultures such as North America, Western and Eastern Europe as well as Australia, women were more successful when attempting a short-term poach.   
It is the severe dopamine hit similar to what problem gamblers have, that allows poachers to come back for more. Psychological studies show that mate poachers have severe insecurities in combination with narcissistic tendencies and an uncomfortable dissatisfaction the moment they get what they want. This again, goes back to the rush and dopamine hit their brain gets from "winning" the competition with another person who is noted as a 'loser' in their brains. 
This isn't however a win/lose situation. The situation is just as bad for the poacher/poached and even the other involved such as family. The article Once a Poacher Always a Poacher? Mate Poaching History and its Association with Relationship Quality was published in the year 2020 and note that poachers often report "low quality" relationships despite being successful in their "hunt".
Statistically, relationships with infidelity show instability in terms of a deeper damage such as increased anxiety and depression in both partners as well as the possibility of the dissolution of marriage that leads to a cycle that is hard to break. Children from homes with infidelity may have a less stable version of what they want in life. We seek what we believe is comfortable. Infidelity is also known in psychological sciences as a possible trigger for domestic violence. 
Whatever the justification may be, science has shown that cheating leaves a lasting impact on both the cheater and the cheated. It causes damage that takes only time to understand.