You're using the relative minor to the major. Thus if the song is in G major, your singing the E minor. Both use the same notes, what changes is where the resolution lays.

That being said: Can't Buy Me Love the chorus is in E minor. The Verses are in the relative major of G major. They occasionally change to some 7th chord structures and modify the melody accordingly. Thus you're not singing a minor melody over major chords, you're singing a minor resolution over the minor part and a major resolution over the major part.

Jody Whitesides
A Funky Audio Lap Dance For Your Ears!
www.jodywhitesides.com