We all have a fetish, I´d say most of us probably more than one. Sometimes a fetish can be considered as weird or wrong, but what one person might consider weird or wrong the other might consider as normal or acceptable. So don´t be suprised if you meet someone that has a fetish that you don´t like and something you don´t want to get involved with.
So how do you deal with that? Well the best thing you can do is be honest, don´t do something you don´t want to or something you consider as wrong. Because it´s better to be honest about these things, the longer you pretend the more you´ll dislike it and that could cause friction in a relationship. But it´s not nice to call someone weird because you don´t like his or her fetish, instead you could talk about his or her fetish and find some common ground. For example a guy that is into strap ons and his girlfriend doesn´t like wearing one, then his girlfriend could use something else to still give him his anal pleasure.
Can an uncommon fetish cause trouble in a relationship? It´s possible, but I would assume something in advance just like that. Unless you know that your fetish isn´t accepted by a lot of people, then it´s safe to assume that someone might not like it. So do you tell your partner about it? If it´s really something disturbing or weird then you might want to keep it to yourself, especially when you know your partner isn´t into something like that. But in a good relationship you can tell your partner a lot, there is no shame in a relationship where you love and respect eachother.
I would really advice to find something else if your partner isn´t interested in your fetish, maybe there is something else you like or something similar? Just talk with your partner and hopefully you can find something you can both enjoy. There are no guarantees though, which means revealing a certain fetish might cause problems in a relationship. But it´s better to be honest about something you consider important than to live a lie and pretend everything is good when it´s clearly not. Besides wouldn´t it be better to spend your time with someone that has the same fetish as you or has an interest in your fetish? That´s not a requirement for a relationship, but it will definately help make a relationship stronger and intenser!