According to a new 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
So there is 20% of people who were involved while they started their new relationship. Just add the "people who broke up with someone then a few days later started up with someone" on top of that. Think it's not at least 30%?