Except loving his stories in Othello, why did Desdemona marry Othello?
In Othello, Shakespeare presents two different Desdemonas: the one in Act I is a rebel, defiant of her father; the one in Act II is overly-compliant and passive toward her husband, martyring herself rather than defying him. So, Shakespeare gives us a vixen and a victim, and so it is difficult to tell which Desdemona married Othello for his stories and which married him to spite her father.
The Desdemona of Act I seems to be attracted to Othello's "otherness." She is white; he is black. She is Christian; he was probably a pagan. She is young; he is old. She may well have been a virgin; he, no doubt, had experience. All of this would have been done to spite her father, who--from his dealings with Othello at least--is controlling and manipulative. Her elopement may have given her a sense of adventure, freedom from an otherwise "under lock and key" status in a senator's house.
Brabantio senses her deception, as do other men. They realize she is good at tricking them. As soon as Brabantio is told that his daughter has eloped, he says as an aside:
Later, to Othello, Brabantio says:
Iago even says:
It seems that Desdemona has a rebellious streak once in Venice, but once she gets alone on Cyprus, away from her father, she might have gotten in over head: her rebellion backfires. Suddenly, she is a toy to men (Iago and Othello), who use her as collateral damage. Rather than speak out, as Emilia does, she plays the part of a "good wife," by saying nothing, even on her deathbed.
Shakespeare leaves her character dubious in terms of motivations. As such, she can be read more than one way. As a vixen at least, I believe, Shakespeare wants us to see an undercurrent of sexual attraction to Othello. She would never express this aloud in the play, nor would it have been permitted on the stage regardless. So, I'm afraid what she doesn't say--in terms of motivation for marrying Othello--is more important than what she does say.
I differ a bit from the first answer in my understanding of Desdemona, though I agree that we see her almost entirely through the lens of other people. These others are primarily men, they have their own agendas, and they stereotype women. Desdemona's father, for example, is miffed that she isn't entirely "faithful" (i.e., obedient to him) and so warns Othello that Desdemona might not be sexually faithful to him. How he leaps from a daughter wanting to marry for love to sexual infidelity only shows how obsessed the whole male culture is with female sexual purity. It doesn't mean that Desdemona has a problem: it means she is subject to a society that assumes a woman is likely to be guilty of sexual transgression. Shakespeare knew well that women were fully human and intelligent, so we could read the play as a condemnation of Venetian patriarchy for its narrow vision.
Desdemona, herself, doesn't explain to her father why she loves Othello, but merely implies it is the normal order of things for a woman to shift her affections from her father to a spouse. It's traditional. It's normal. Her mother did it. Desdemona therefore wonders why what she has done is a problem:
And so much duty as my mother show'd
To you, preferring you before her father,
So much I challenge that I may profess
Due to the Moor my lord.
To the Duke and Senators she gives an explanation: it's not simply that she loves Othello's stories, but she falls in love with what they reveal about his soul. As she states, it is not his face (visage) that attracts her, but the quality of his character and mind. This may well be a rebuke to the other men, who seem to see her solely as a sex object:
the very quality of my lord:
I saw Othello's visage in his mind,
And to his honour and his valiant parts
Did I my soul and fortunes consecrate.
Desdemona is a smart, self-possessed woman, as we note in act 2, scene 1. She sees right through Iago, responds to his negative stereotypes about women scornfully, and tells Emilia not listen to him, even if he is her husband. No wonder Iago wants her silenced.
We have every reason to believe that Desdemona picked Othello for his character, and certainly not for spite. Unfortunately for her, Othello fell prey to his own insecurities.