A 1907 article on cowboy songs has the following. (If anybody cares about the precise ref., just ask.)

"Then there's 'The Bucking Bronco.' It is all about a maiden cruelly deceived by a bold, broad-hatted 'buster,' - a dashing, reckless, gay Lothario, who lightly loved and rode away. I am sorry I cannot give the whole song, because it is somewhat 'scandalous' in places, but it begins like this:

Oh, my love is a rider,
    Broncos he does break;
But now he's quit riding,
    And it's all for my sake.
Oh, the first time I saw him
    Was early in the Spring -
He was riding a bronco
    That highly did fling.
He tipped me a wink
    As the bronco did go,
He winked ---------- etc., etc.

"I've yet to see the cow puncher who doesn't know that song. There are variations of it, here and there - all of them pretty candid and vigorous - but take it by and large, it is the standard romantic ditty of the plains."

Hmm. "Somewhat scandalous in places," "candid and vigorous," but "romantic."

It sounds to me as though he's thinking of a typical double-entendre song of the "Soldier and the Lady" ilk. The familiar "Bucking Bronco" tune even resembles it.