Intellectual gain can be marrying for prestige or intellectual companionship. Each of these reasons can be specifically defeated. Nevertheless, more generally, one argument can defeat all these. Idealy a person follows the commandment only for the sake of God and regardless to all personal effects. Nevertheless, the ideal is not usually possible.
People have complex notivations that can be both conscious and unconscious.
The simple answer comes from the Writings. We can take this statement at face value and endeavour to marry, just as we endeavour to accomplish all our other moral obligations. Unsuprisingly though, this simple answer leads to many more questions. So again we must turn to the Writings to explain the answer that has been given to us. From this statement, we obtain a clear understanding of the context of all the laws and principles elucidated in the Baha'i writings.