Sexual Desire has mainly physiological and psychological components. If it is right from the beginning of your relationship then possibility of psychological reasons are more likely. In-depth analysis of her rearing, sexual orientation, childhood sexual experiences (sexual abuse) etc is required. She may be suffering from Hypoactive Sexual Desire Disorder. Psychiatric evaluation can throw light on this. Moreover you may think of getting her Thyroid Profile as a rare possibility Hypothyroidism may be the reason. Although having two children after her low desire rules out any major hormonal problems.