Imagine that every time you fantasize about another woman, your woman starts fantasizing about someone other than you. Ask yourself what these other women have done for you in comparrison to yours. If you had a time when you were sick as hell and your woman took care of you, think about that and ask yourself if she would still be there if she knew what you were thinking or if you acted on those thoughts. You're a human being and there are going to be times you think of other women while your significant other will think of other men(or women). Thats justa part of life and our biology, but we can choose not to act on these biological attractions.