The Internal Revenue Code (the Code) contains numerous special rules applicable to the income taxation of persons related by marriage, birth, adoption, or ownership. This Article suggests a new approach to their analysis. Many basic tax rules assume that taxpayers are self-interested and unaffiliated. Where this assumption is incorrect, the Code makes adjustments to its otherwise applicable rules. Most of the resulting related-party antiavoidance rules apply only in the context of specified formal relationships-marriage, parent/child, or owner/business. The Article tests this thesis by comparing the income tax treatment of heterosexual married couples with that of gay couples in committed long-term relationships. Gay couples are not married for tax purposes, nor are they spouses within the meaning of the Code. Gay marriage therefore never by itself invokes any related-party rules. The Article explores a series of tax avoidance problems in the contexts of marriage and extended families. None of the relevant anti-abuse rules apply to gay spouses. As a result, gay couples should be able to arrange their affairs so as to pay federal income tax at significantly lower effective rates, on average, than identically situated heterosexual married couples. The Article concludes that the only way to ensure that gay couples will be taxed no more favorably than heterosexual married couples is to list gay marriage as one of the proxy relationships that automatically invokes pertinent anti-abuse rules-in other words, to treat gay marriage as marriage for federal income tax purposes. In the absence of an attractive formal status that then invokes related-party anti-abuse rules, well-advised gay couples are, and will continue to be, permitted to pay systematically lower federal income taxes than heterosexual married couples--a result unlikely to be acceptable to a majority of Americans in the long run.