The male sings Luka's part because people just assume she's the top because she's older/taller/etc. But she actually has lyrics like "You've got me wrapped around your finger", "Shackle me, make me feel how much you need me" etc. Miku's lyrics aren't dominant though, so I guess Luka was presented this way in Magnet to restore some balance and have them more as equals than dominant-submissive.