Your judgement is wrong.Its not only girls,but also boys who can leave their girlfriends for a better one.If the other one is more beautiful or richer,they may tend to be attracted to the other.Its just humans' nature.they will move where they find more happiness and comfort.And,marriage is a life-long journey.you must be wise enough to choose a correct partner.Its's said that "marriage can complete you,if you get a good partner.otherwise, you would be just screwed"
Ejaz_D_RoxTar