How can romantic love ever be unconditional love?

People get into relationships for reasons such as the other person's looks, personality, status, money, among other things. All of these are temporary, but to love someone unconditionally would mean to love someone without conditions or reasons meaning that you would want that special someone to be happy even if it means that person won't love u back or has physical relationships with other people, correct? So, why do people who say they love whoever with all their heart still want that someone to only be theirs and theirs alone? Is this not selfish? I'm just interested in hearing your thoughts.