I'm no legal scholar, but it seems to me pretty obvious, and the decision could have been written in a single paragraph rather than a hundred or so pages: same-sex marriage is constitutionally protected for the following simple, clear, and inarguable reasons:
1) Equal protection under the law for all citizens is at the core of our Constitution and of American values.
2) Sexual preference is not a matter of choice any more than is color of skin, and, therefore
3) The only basis for considering homosexuals undeserving of equal rights is a religiously-based one and, therefore
4) Since the Constitution forbids raising religious law above civil law, there is no Constitutional basis for preventing same-sex couples the right to marry, and, also
5) No state has the power unilaterally to remove Constitutionally-guaranteed rights.
6) Period.And, oh yeah: No one is forcing anyone to marry a person of the same sex, nor will any given house of worship be required to perform such a marriage. Also obvious.
(Added: As moving as the final paragraph of the decision is, it's only the last couple of lines that are legally relevant: "They ask for equal dignity in the eyes of the law. The Constitution grants them that right." See? Simple.)
(More: also obvious: preventing same-sex marriage harms an innocent group of citizens. Allowing it harms no one.)