I think there should be an understanding and respect between the partners, be it husband and wife or boy friend and girl friend, in terms of sex. I am not married, but I think sex is not the "only thing" a man and a woman become together for, although it may be one of the significant things and one of the important things to keep a marriage life healthy, happy, and running. There is ofcourse love and respect in a true relationshp and because of this, while having sex, if one of the partner is not comfortable using mouth (or any any of the sexual positions) for sexual fantacies, the other partner should honor that. If he/she fails to do so and forces the other partner to  go against his/her will for sexual fulfillment, then, there is no true love or respect among them, because sex is not the only thing in life.