Marco doesn't love Jackie. He loves the idea of Jackie.
Marco will fall in love with Star when star learns to be more responsible, which is why she was sent to Earth in the first place. Right now, Star relies on Marco to support her and be the responsible one. Once Star matures, Marco will see Star in a new way. But not before.
It has all been foreshadowed many times in dream sequences through symbolism (freudian balloons, red(love)/green(jealousy) colors, the themes of previous queens (shyness/ bravery, sacrifice/selfishness), mother issues, control vs freedom to grow, Marco's fear of being held back by Star vs fear of letting her explore, list goes on)
TL;DR Starco is endgame, because it requires Star to grow to be worthy of conquering Marco.