To me, it seems a lot like my atheism. I'm an atheist because it just makes more sense to me that there is no God.
Likewise, if I'm in a relationship (which I am), and I develop feelings for someone else, it doesn't make sense to me that I should be forced to choose to pursue only one of those relationships. If I end up being in love with two people, or three or however many, then why shouldn't I follow my heart? 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?