"The secret to writing bubblegum rock songs was to make them major 7th so that they sounded sweet and non-threatening."

It depends, josepp, on what you mean by "Bubblegum Rock", but "Classic Bubblegum" of the Ohio Express/1910 Fruitgum Company variety used straight major and minor chords and *avoided* major 7ths. The "SECRET" of Bubblegum music was the pulsating bass-all eighth notes, on the chord fundamental, while a cheesy Farfisa organ played something like "Bah Bah Bubah-Bah Bah Bubah" and the guitar avoided leads of any kind.