I think the biggest reason for the decline in heterosexual marriage is because women are finally getting better options. No longer do they have to rely on some loser man to have a stable and secure life, and they don't want to submit themselves to patriarchal nonsense. Women don't have to marry someone in order to get by, so now they aren't settling for partners that don't cut it. Much of "traditional" marriage has always been bad for women. For much of human history, it was literally sex slavery, where a woman was sold to a man by her father, and could be beaten into submission if she didn't want to have sex with her new husband (whom she had no say in choosing) and have his children. In this country alone, husbands could only be prosecuted for raping their wives since the 1970s. That's the marriage that's in decline. The marriage where women had to submit to a man in order to have a roof over her head, where her fortunes were never in her control, but were in his instead, and where she had the right not to have sex for the last 40 years (or as little as 21 years if you live in states like Oklahoma or North Carolina, or in South Carolina where marital rape prosecutions are very difficult to obtain at all, making it effectively legal today).
But, you know, it's because people are too "egotistical" now and don't love god enough. Funny how the Christian myth involves god raping a woman and forcing her to have his child, isn't it? Sounds like the real change is that women aren't forced to give up their own wants and choices and subordinate them to a man's choices anymore. Because a woman being as willful as a man is wrong, right?
Just like blacks wanted additional validation for their schools as being the same as those sanctioned by the white majority. Seriously, why do you need to attack the motives of gays in order to make yourself feel better? Separate but equal is unconstitutional. That means your civil union nonsense (which backwards religious schmucks only now cite as a compromise because it's too late, you fought tooth and nail to stop civil unions originally) is not sufficient. And for all your whining about religious marriage... tough. You and yours failed in whatever custodianship you ever had over society. We, the enlightened secular people, are making a better society than you ever could. And we will not ever let you drag it back down again.