Says right in the article:
They're stirring up votes so they don't lose in the upcoming elections. that's why the FMA is timed carefully to compensate for the biggest popularity drop for a president aside from Nixon immediately before impeachment.
Even for those who don't want the amendment to pass, they're appearing as the party of values. Having some fixed and inflexible rules can be appealing in tumultuous times. They come off looking as principled when the Democrats look scattered.
And its always easier to attack another group of Americans than take on the big issues.
Here's the amazing thing:
"Ages of experience have taught us that the commitment of a husband and a wife to love and to serve one another promotes the welfare of children and the stability of society." --GB
So? I totally agree but this has nothing to do with the FMA. This phrase is a justification to strengthen marriage in other ways, discouraging hasty marriages, discouraging unmarried childrearing, discouraging divorce where possible. The FMA doesn't promote heterosexual marriage in the slightest. And if John and Susie lose interest in being married just because they failed to prevent their neighbors from doing the same--well, they obviously never cared much about it in the first place.
It's just an exclusive club that nervous members want to keep private. I'd have more sympathy if they actually had to hang out with gay people on their private golfcourse, but they don't.
My solution remains: Get the government out of marriage by letting churches marry whichever consenting adults they want. And refuse those who they don't want to marry. And keep religion out of unions by recognizing partnerships separately, allowing people to form their households and partners without regard to race, sex, age, wealth, religion, and so on.