Regardless of how its popularly defined, the word technically means spreading the Gospel and good news about Jesus Christ. Thus, all Christians are would technically be "Evangelicals", not just fundamentalist protestants.
>> Still, only about one in four gave the “Bible answer” of “definitely not,” and close to half are planning on or leaning toward it. <<
Sounds like they believe in the "Biblical" definition of marriage the same way they believe in the "Biblical" definition of Holy Communion. Maybe 1 in 4 of them use bread and wine like Jesus did, and actually believe they're receiving Jesus. The rest of the so-called "Biblical" evangelicals use grape juice and Ritz crackers and think its "merely a symbol".
It's not surprising to me only about 25% of "Biblical" Christians actually follow what the Bible itself says. The rest seem to think "Biblical" means having smog machines, light shows, jumbotrons, and rockin' out to Hillsong music on stage.