What sort of relationship does Cassio have with Bianca?

Iago describes Bianca, in the play Othello, as "a housewife that by selling her desires / buys herself bread and clothes." Essentially, Bianca is a prostitute, who, Iago says, "dotes on Cassio," while Cassio himself cannot hear of her without smiling. Cassio does seem extremely fond of Bianca—he makes her a gift of the handkerchief, and speaks to her gently, calling her "my sweet." However, he is evidently aware that her reputation could have a detrimental effect upon his own, as he asks her to leave before Othello should see him "woman'd." When Bianca asks why, he is vague, but Bianca understands the situation, saying, "But that you do not love me." Bianca seems to accept this, although she asks Cassio to promise that he will come to see her soon.
The next time Bianca appears, we see clearer evidence that Cassio's sweet talk toward her may not be entirely genuine, as he demands to know the reason behind her "haunting" of him—that is, complains that she never leaves him alone. Arguably, Cassio simply does not want Othello to know the depths of their relationship. Bianca, in her turn, is angry that the handkerchief Cassio has given to her is "some minx's token" and believes that Cassio has another "hobby horse," or mistress. But we know that her feelings for Cassio are strong, as expressed in her lamentations over him when he is wounded.