I think you might be referring to the Obergefell decision? If so, the Court didn't invent marriage, which is a contract that can only be entered, maintained, and dissolved in accordance with state law. The right to marry, which is a fundamental right protected as a liberty interest under the due process clause of the Fourteenth Amendment, is subject to reasonable state regulations. For instance, states may legitimately determine the age of consent and prohibit marriages between closely related people.
When you have two groups of similarly situated people, and one group is allowed to exercise a fundamental right and another group is prohibited, then conflict arises. The institution of marriage confers many tangible and intangible protections and benefits upon marriage partners and their children. Members of the group who are denied the right are entitled to petition our courts for redress of their grievance and to seek equal protection under the law. There simply was no reason why same-sex couples should not be afforded the same (equal) rights as heterosexual couples. That's just a straight forward application of Constitutional Law 101, not an expansion of power.
All the Supreme Court justices, past and present, understand that our country did not stand frozen in time when the Constitution was written or when the civil war amendments were adopted. For instance, in
Brown v. Board of Education, the Court stated the following:
"In approaching this problem, we cannot turn the clock back to 1868, when the Amendment was adopted, or even to 1896, when
Plessy v. Ferguson was written. We must consider public education in the light of its full development and its present place in American life throughout the Nation. Only in this way can it be determined if segregation in public schools deprives these plaintiffs of the equal protection of the laws."
I apologize for going off track, but I don't agree that our guiding constitutional principles and case law cannot be applied to new fact situations that are present today but were not present at an earlier time in our history.