Inheritance should be simple enough with normal contracts, regardless of marriage (same goes for hospital visitation and medical decision-making, which you didn't mention). Immigration is already completely arbitrary, at the whim of the government. The government could change the immigration effects of marriage just as easily as changing who can get married. That brings us to taxes. Well, that's already an extremely obvious violation of the equal protection clause. Why should married people's taxes be any different than single people's taxes?