The Supreme Court on Tuesday invited a Massachusetts lawyer to come argue that the justices cannot rule on one of the gay marriage questions it had planned to decide next year. The court asked lawyer Vicki C. Jackson of Cambridge to join the gay marriage arguments this spring, but she won’t be arguing whether it’s legal for governments to treat gay Americans differently in issues of marriage. Instead, at the court’s invitation, Jackson will be arguing that it’s improper for the Supreme Court to even consider making a ruling on a federal law that treats gay married couples differently from heterosexual married couples. The high court will be hearing two gay marriage arguments: first, whether California’s constitutional amendment that forbids same-sex is constitutional. The second question is the one Jackson will argue that justices should stay out of: the constitutionality of a federal law that denies to gay couple who can marry legally the right to obtain federal benefits that are available to heterosexual married couples.