It's true. One of my friends, 3 year relationship, they'd had heaps of ups and downs like any relationship. But otherwise never argued or had problems.
One weekend he was being an extra bit of a prick, cause he was tied up with work. Following weekend he finds out she went to dinner with guy she knows from work to chill out, and he ended up seducing her and she left, from that point on she never looked back.
New man = easy to forget about the old one. Fact.