There is nothing in the human makeup that say we can't love more than one person romantically.
@alex117 there is nothing that says you have to love them equally. Let's for a moment take it away from romance, say children, can one person love more than one child? Can they love all their children to their fullest potential or is love limited that any "love" spent on one child is love lost to another? Now could I love more than one women romantically, sure would they be treated exactly the same? Probably not, but that doesn't mean I couldn't love them to their and my full potential.