For men it is true because men get tired of a single women sexually generally. Men like variety, something new, different size titties, feel of her pussy, etc.....
Girls, IMO, are into taking shit. They feed off of your plate (food, money, life) and take your shit, spiritual or physical. So they are looking for the best provider (of sex, money and fun) which men differ qualitatively in their ability to provide. Women do these little test to find out how much you will provide them and even if you have bank, etc......but you will not give it to them, they look for a different provider.
Men give. They are like Dicks
Women take. They are like Vaginas
Men like variety in women and get bored with one girl.
Women want security and they get this through one man.
Hope that helps Bro.