Your imaginations come into play when you are having sex, especially if you are not sharing a bed with anybody else. If you're fortunate and both of you are willing, your fantasies will eventually come true. Consider it this way: If you two truly love each other, she will have you in mind when she imagines that other man slapping her from behind as she leans over the back of the couch—or, should I say, your dick in her mouth—because we all have fantasies like that. The issue arises when she imagines fantasies without you. Therefore, don't give up and don't be angry about it. Embarrass it, assist in making it a reality, and then revel in it as if it were intended to be.