The State cannot demean their existence or control their destiny by making their private sexual conduct a crime. Their right to liberty under the Due Process Clause gives them the full right to engage in their conduct without intervention of the government. “It is a promise of the Constitution that there is a realm of personal liberty which the government may not enter.”
Now that was on legalizing Homosexuality itself and not marriage, however.
Conservative Justice Scalia said in the same Case " This reasoning leaves on pretty shaky grounds state laws limiting marriage to opposite-sex couples."
Consenting Adults is used repeatedly in the Court Decision, not children