I feel the same way as you, but....
Originally Posted by Tiberius
After all, people can have several best friends and none of those friends feels like they are being cheated on. Parents have more than one child and the children never feel like they are being cheated on when the parents give a bit more attention to the sibling. Why should romantic love be any different?
Both friends and children can feel extremely jealous and cheated on in the exact situations you mention. It's just that we're taught that that jealousy is bad, while romantic jealousy is "normal and means you care".