While this is true in the very basic Psych 101 sense, it does not hold true for every single relationship that has ever existed. Quite often people find temporary mates to satisfy a temporary need.
Seems to me that they are both satisfying a need. She gets some young sex with a dude that probably provides things Hulk coudln't (young innocent fun and a hard cock). He gets money, fame and fortune.
Seems pretty simple to me. Not sure the sweeping generalizations you made apply here.