Canon 160 states that all marriages should be assumed valid until proven otherwise.
Actually, it is Canon 1060. And, it does not say “all marriages should be assumed valid until proven otherwise”.
What it does say is this:
Can. 1060 Marriage possesses the favor of law; therefore, in a case of doubt, the validity of a marriage must be upheld until the contrary is proven.*
So, for example, a marriage outside the Church without dispensation does not possess the favor of the law, because there is no doubt concerning its validity. While a marriage celebrated in the Catholic Church with all proper paperwork, etc, must be presumed valid until the contrary is
proven.
However, if one of the necessary elements of marriage is clearly missing, e.g. openness to life, intend to be faithful, intent to love, free choice of one or both parties, is it ok to not assume this?
No, because it is not “clearly missing”. When two people exchange consent it is presumed that the words they say and the actions they take externally and their intent/disposition of the will internally are one and the same.
Can. 1101 §1. The internal consent of the mind is presumed to conform to the words and signs used in celebrating the marriage.*
So if they say in the vows that they take X until death do us part, they would have to prove in the external forum (i.e. through witnesses and testimony) that they did not in fact intend what they said and that a defect existed at the time of the marriage.
One may not simpply “assume”.
Say that a young woman is forced against her will to marry
This would have to be proven, it cannot be assumed.
to marry a man who has shown by his actions that he lusts after her, rather than loving her.
This is irrelevant to whether or not the marriage is invalid, and it would not render a marriage invalid. “Love” (romantic love) is wondeful, but not required for a valid marriage. People have validly married in arranged marriages for thousands of years.
(As in the case in a book I’m reading)
Novels are never a good place to try to get one’s theology of marriage, nor to apply canon law.
I don’t see how I could assume that this marriage is valid without being dishonest with myself?
Um, it is a novel. I am unsure how dishonesty with oneself enters the equation.
How would the canon law apply in such a situation, were the situation to be real? (As I assume it is for someone somewhere, likely in Saudi Arabia)
Case by case. We can make no speculation regarding any marriage.