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:
O she deceives me
Later, to Othello, Brabantio says:
Look to her, Moor, if thou hast eyes to see:
She has deceived her father, and may thee.
Iago even says:
She did deceive her father, marrying you;
And when she seem'd to shake and fear your looks,
She loved them most.
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.