If you have a sexual relationship where one person has a fetish and the other isn't okay with it, then you have a problem in the relationship. If you're in a relationship and you have a fetish you haven't disclosed, and you are finding it hard to enjoy sex because of the lack of disclosure, that is also a problem.
In both cases it may not be the fetish that is the problem. It might be miscommunication, or different sexual values, or trouble with communication in general. There's no simple answer to the question, is a fetish bad for your sex life, because even if the fetish feels like it dominates your sexual thoughts and feelings, as a sexual being you are more than the fetish. As such, what makes your sex life good or bad is bigger than just your fetish.
Here are a just a few ways that a fetish could, conceivably be good or bad for your sex life.
Keeping it hidden: If you have a fetish and are keeping it hidden because you feel shame about it or are afraid to talk with your partner, that may be having negative repercussions on your sex life. Your partner may feel like you're holding back but have no idea why. You may begin to feel resentment for not being able to be "yourself" with your partner. Sometimes we keep things to ourselves by choice, because we choose not to share. But if you are silent out of shame, embarrassment, or fear, it can have a negative effect on your relationship and sex life.
Compartmentalizing the other: A common argument as to why fetishes are bad for relationships is that they involve focusing on one thing to the exclusion of other things. When you fetishize something or someone you can't appreciate it (or them) in their entirety. So fetishes make sex less human, less about the connection between two people and more about one person "getting off". Of course this isn't necessarily the case. But if your partner feels this way, or if your fetish is something you use to avoid connection or intimacy, then if you want the relationship to last, it's something you both have to talk about and negotiate.
Making yourself vulnerable: Revealing a fetish to a partner is always a risk. You are exposing something that is powerful and personal for you, and in doing that you are sharing part of yourself with your partner. Regardless of their initial reaction, this can take your relationship to new depths of intimacy and sharing, and may allow your partner to talk about the aspects of their sexuality they may find embarrassing or shameful.
Talking it out: In order to talk with your partner about your fetish you have to have pretty good communication skills. Sometimes when someone first discloses a fetish it doesn't go well because good communication ground rules aren't in place. But if you get through the bumps and manage to negotiate a sexual relationship that is mutually satisfying, you are learning good communication that will serve you well in this and any other relationships.
Being sexually selfish: Another complaint one hears from people who don't have fetishes about those who do is that they are sexually selfish. They are only interested in their own pleasure and by focusing on a shoe, or an arm, or some other body part or object, they dis-empower their sexual partner. This may be the way some people feel, but again, it doesn't have to be the case. Stating a strong sexual desire or preference doesn't have to mean that you aren't also interested in your partner's sexual pleasure and happiness. A fetish doesn't have to be about one person getting their needs met at the expense of the other. If that's what it becomes, then it clearly is a problem in a sexual relationship.
Celebrate sexual creativity: For some people there is a great deal of creativity in the experience of a fetish. If a fetish can be integrated into your partnered sex life it gives everyone involved the opportunity to open up the possibilities of what your sex life involves. The simplest example is people who believe that sex is just intercourse will have to seriously expand their definition of sex, if they want to include fetish in their sex life.