Are fetishes harmful to a married couples sex life?
If so, how do you get rid of them?

Because I do not treat sexual issues, I have never had such a case come to my attention. So, my answer is going to quite general and limited, not quite what you might have hoped to read.

Let us start by clarifying what is a fetish. It is the use of an inanimate (non-living) object to sexually stimulate a person. In psychology, we usually expect men to use female clothing, especially undergarments, for this purpose, but it certainly is not limited to those. One's imagination sets the limit as to what might be used. In order for this behavior to attract clinical attention, the behavior must be causing significant personal distress or impairment in some public area of life such as employment or socializing. Without the use of a fetish, sexual arousal  might not be possible or sustainable. To that end, the client might require his/her lover to wear the particular item during foreplay and intercourse.

To answer your first question, I would say a qualified "yes." Qualified because neither partner might be aware of the deeper and less obvious effects of fetishism. Until those deeper effects present themselves in some objectionable way, such as lack of genuine intimacy or the feeling of being treated like an object, both partners might think that all is well. What little I know about these leaves me with the impression that the effects would be nearly the same as those which pornography has on a couple. It does not take a rocket scientist to soon question, "Is it me or the object that excites him/her?" When pornography is an issue, it is not uncommon for the man to use the woman as the physical object upon which he projects the pornographic image which is actually responsible for his arousal. So, if you consider the image as a non-living object that is mentally superimposed (laid) on the woman in order to become and stay aroused, you might see the connection that I do between the two.

Because of that perceived connection, if I had to treat this in a man, I would be strongly motivated to offer him my sex addiction treatment plan, making whatever changes would be necessary to customize it to his situation. However, you would be wise to consult a sex therapist with these questions to see what is currently proving effective in their field of specialty before acting on my observations.