Benefits for unmarried partners are being offered by more businesses. Just this year, the percentage of Fortune 500 corporations offering benefits for domestic partners exceeded 50%. As the benefits are becoming more common, companies are struggling to establish consistent definitions of “domestic partners.”

Raleigh-based Progress Energy will begin offering domestic-partner benefits next year. The company expects 200 of their 15,000 employees to apply. Companies like Progress Energy that have made the decision to offer the benefits say the policy gives a competitive edge in hiring and reflects a culture of fairness and acceptance. Companies that choose not to offer the benefits say that they are too costly and controversial.

Progress Energy’s new program is designed to give the same benefits to unmarried couples –whether gay or straight, that married couples receive. The company’s policy requires that the unmarried couple have lived together in an exclusive relationship for at least six months. The benefits are designed for lovers in committed relationships, not for friends or transient relationships.

One problem is that the rules are not consistent from employer to employer. Some companies require partners to live together for at least six months, others require a year. Some employers, like Duke University don’t require couples to live together at all. Duke is trying not to discriminate against those who have partners that might work in different states, since married partners are not required to live in the same household to have coverage. Progress Energy and other employers specify that the relationship must be exclusive and monogamous in order to qualify. Other employers don’t make that requirement.

The proofs required by various companies vary as well. At Progress Energy, couples just need to sign an affidavit that says that they are partners meeting the company’s criteria. Other companies require documentation like joint bank statements or mortgage agreements. Further complicating things, some employers do not extend the benefits to unmarried heterosexual partners. Duke University doesn’t offer the benefits to straight couples because they have the option of marrying. Other organizations offer the benefits to gays and straights alike, not wanting to discriminate based on sexual orientation. There are other disparities as well. Domestic partners have to pay tax on the value of their benefits, but married couples do not.