In my opinion, it is not apathy but cowardice. Apathy just plays better. One gets spat upon less frequently on high ground.
And of course there’s a lot of ignorance out there that needs to be disguised, too.
Case in point:
There are two reasons why this issue has ended up before the U.S. Supreme Court, and they relate to the two separate cases that have been appealed to this point: (1) the U.S. Internal Revenue Code includes distinctions for "married couples" among taxpayers, and (2) the Defense of Marriage Act of 1996 is a Federal statute with legal ramifications for "married couples."
I'll let the first point stand on its own. The Internal Revenue Code is a massive pile of nonsense that even the IRS's own employees can't decipher correctly. At its heart, the first case is basically a tax dispute, brought by a lesbian plaintiff who is questioning why she (or, more accurately, her lesbian partner) had to pay a $300,000+ estate tax bill when her "partner" died, while her neighbors who are recognized as "married" under the tax code would not have been required to pay this tax. Forget about married couples and lesbians in this case. The real argument here is why anyone should be compelled to pay a $300,000 tax bill to the Federal government after he or she has deceased. The "right" side of this political argument was lost the moment we accepted the fact that the Federal estate tax -- which happens to be one of the platforms of the Marxism -- is a legitimate form of taxation. Anyone who stands up before the U.S. Supreme Court and renders arguments based on the status quo under Federal tax law has already capitulated on this point.
The second point relates to the legitimate role of the Federal government in marriage. Many of us predicted that the Defense of Marriage Act (DOMA) of 1996 would be overturned the first time it came before the Supreme Court, since there is no basis under the U.S. Constitution for the Federal government to have any involvement in marital law. Anyone who argues the "traditional marriage" side of this issue on the basis of the DOMA is on the wrong side of Constitutional law.
The larger issue here is that a lot of principled people have no interest in this battle because it's completely alien to them. For a lot of these folks, a legal battle over the rights of people to engage in homosexual behavior and over the need for governments to provide legal recognition for these people may as well be an argument over the Constitutional rights of aliens from Mars, or of turds in the toilet. That's how bizarre -- even completed 'effed up -- homosexuals are to them. How much interest do you think you'd get from conservatives if the U.S. Supreme Court was considering a case involving a plaintiff who wanted Federal protection for his/her right to eat a railroad tie every weekend?