When you sentimentalize marriage, you ruin marriage. Love does not make a marriage. Marriage is a distinctive type of relationship that has a distinctive type of origin. What makes a marriage must itself be distinct. You can have true love between friends, brothers, parent and child, teacher and student, etc., but that does not make them married. Human affection makes marriage fun, sweet, beautiful and romantic. But human affection does not make a marriage.
I saw a bumper sticker that said, "I love love. Legalize same-sex marriage."
If marriage was nothing more than a romantic union of two (or more) people, then yes, we absolutely should let gays get married to one another. When the debate is framed in such a way that it really all comes down to "Are these people capable of loving one another?", then of course gay marriage is an inevitability.
The reality is we have been basing marriage on human affection for the last century, and that, more than anything else, has destroyed marriage.
Why? Because subjective emotions are not enough. If marriage is the union of two people, then basing that marriage on one's own inner emotional state cannot suffice as the foundation. I cannot feel your emotions. You cannot feel my emotions. They are inherently subjective. Surely, I can relate to your emotions, especially when I compare them to my own experience. But to try build a stable union of two lives on something that is necessarily individualistic and subjective is to build upon a foundation of sand.
And that is what America has done in straight marriages since at least the 1930s with the rise of contraception and no-fault divorce. Chesterton once quipped that the rise in no-fault divorce would not lead to frivolous divorces, but to frivolous marriages, like Britney Spears' 24 hour marriage.
When we view love as something you "fall into", a seemingly unconscious accident glorified by our technicolor storytellers in every dang Rom-Com out there, it is equally easy and unconscious to "fall out of" love as well.
Love does not make a marriage. It makes divorce. If that is all you got.