Interestly, one in five couples get together through 'mate poaching', according to the study reported
here in Psychology Today:
Wedding announcements in newspapers rarely tell the whole story. If they did, they would occasionally read something like this: "The happy couple met through her boyfriend at the time, who is the groom's former best friend. It took four months to lure her away."
According to one study, up to 20 percent of long-term relationships begin when one or both partners are involved with others. Evolutionary psychologists call this "mate poaching." This figure holds steady across age groups and among couples who are married, living together or dating, according to psychologists who polled some 16,000 individuals in 53 countries as part of the International Sexuality Description Project. Most surprising to researchers: Sweetheart-stealing is prevalent across continents and cultures, although it is notably less common in East Asia.