It’s important to understand the meaning of “necessary” as Aquinas applies it to Our Lord’s incarnation. When we hear the word “necessary” we usually understand it with the meaning Aquinas calls the “first way” - a way Aquinas specifies “was not necessary”.I answer that, A thing is said to be necessary for a certain end in two ways. First, when the end cannot be without it; as food is necessary for the preservation of human life. Secondly, when the end is attained better and more conveniently, as a horse is necessary for a journey. In the first way it was not necessary that God should become incarnate for the restoration of human nature. For God with His omnipotent power could have restored human nature in many other ways. But in the second way it was necessary that God should become incarnate for the restoration of human nature. Hence Augustine says (De Trin. xii, 10): “We shall also show that other ways were not wanting to God, to Whose power all things are equally subject; but that there was not a more fitting way of healing our misery.”
Regarding an Incarnation if man had not sinned, Aquinas did not rule out the possibility even tho he personally did not believe there would have been one.I answer that, There are different opinions about this question. For some say that even if man had not sinned, the Son of Man would have become incarnate. Others assert the contrary, and seemingly our assent ought rather to be given to this opinion.
For such things as spring from God’s will, and beyond the creature’s due, can be made known to us only through being revealed in the Sacred Scripture, in which the Divine Will is made known to us. Hence, since everywhere in the Sacred Scripture the sin of the first man is assigned as the reason of Incarnation, it is more in accordance with this to say that the work of Incarnation was ordained by God as a remedy for sin; so that, had sin not existed, Incarnation would not have been. And yet the power of God is not limited to this; even had sin not existed, God could have become incarnate.