The benefits are not portable. They could be lost by changing jobs. Partner benefits are subject to federal and state income tax, unlike benefits for married couples. Many companies require unmarried employees to sign an affidavit before receiving benefits. Often, these documents contain statements never required of married partners, such as sharing financial obligations.
Married couples can have prenuptial agreements which separate their funds and ownership.
Married couples only need to know each other long enough to sign a marriage license. Unlike legal marriage, domestic partnership has little legal precedent and future court cases could be disastrous. For instance, on the basis of an employee's domestic partnership affidavit, an individual could be held legally responsible for a previous partner's current debts, whether or not they were jointly incurred. If that happens, we will have secured one of the major responsibilities of marriage with only a few of the benefits. Working for modest benefits can take years of meetings, reports and negotiations.