I disagree with OP's friend. Men DO have an incentive to get married. Many studies have been done on married men and the benefits they get....they rise professionally, economically, socially and healthwise, married men are healthier. They also get to perpetuate their legacy via their children from the marriage.
That aside, when women allow men to have all the trappings of marriage without the marriage, then....there is no incentive. Women need to stop falling for it and giving up everything they have to give to men who are not their husbands.