An alternative explanation would posit that Pancho was weaned all too early and had latent sexual fantasies about his mother:

You weren't your mama's only boy
But her favorite one, it seems;
She began to cry when you said goodbye
And sank into your dreams

There's the additional suggestion that Pancho was also impotent:

Wore his guns outside his pants

Lefty on the other hand (his larboard one) was equally tramatized but by too early toilet training, the key reference in the song being these lines:

The dust that Pancho bit down South
It ended up in Lefty's mouth

This song, I fear, is never going to seem the same after all this erudite discourse.

I suppose, out of kindness, we might have let it go.

Charley Noble