The justices who voted against did not vote against gay marriage - they voted against the court defining or redefining the State Constitution.
This is precisely how constitutional law is developed -- from precedent setting decisions interpreting that constitution. When a suit is brought on constitutional grounds, it is the court's charge to interpret that document. It's been going on for as long as our country has been around. (In England, btw, there is no constitution, just a tradition of "common law" -- that is, judicial precedent.)
The only way to reverse such a decision is to amend the Constitution.
No, the court may hear a similar case on the same grounds in the future, and find differently.
The Federal Court will decide in favor of the decision - they have to because the Mass. judges have said it is part of our Constitution so they can't rule against it.
No, the federal courts can also refuse to hear the case on the grounds that it doesn't fall under the provenance of federal law.
If another state chooses not to recognize it (this will happen - Ohio I believe has already started to prevent recognition) there will be lawsuit after lawsuit after lawsuit.
Yes, there likely will. Like it or not, this is how the process works. It has never been about what is acceptable to the majority of American people, but about how far the reach of the law does and should extend, as determined by whatever judiciary reading whatever document through whatever philosophical (or, sadly, political) lens they care to employ.
The Mass. court has dictated a law that everyone must follow.
This is simply not the case. Abortion laws, to pick up another hot-button issue, vary from state to state, precisely because the federal government determined that said laws should be determined by states and not by the federal government. Of course, there are areas where the legislative and executive branches of the federal government have to deal with that issue, but by-and-large it is an issue for states to work out for themselves. Whether the laws put in effect by each state are in concord with its electorate (and whether they should be), is the responsibility of those states' governments.
That's my piece, anywho. I'm not a law student, the gf is.
(Edit: Roe v. Wade overturned a Texas statute and determined that abortion was federally protected; precedents since then have either expanded or limited the situations in which it is protected, and what determinations are to be left to the state. Sorry.)