Because the sheer amount of fraud that would happen would tank the insurance companies. If you have lived with your partner for years and are practically married anyway, a simple court marriage would give your partner benefits. It has nothing to do whatsoever with discrimination since the vast majority of states don't have a provision for same sex marriages being allowed, so there's a provision for them. You have no such legal handicap and besides, if my partner didn't want to get married but wanted the benefits provided by UPS I'd tell her they're hiring pt right now and get her own.