I had an ex with a pretty standard fetish that made me really uncomfortable. Early in our relationship, I told him I did not want to do that particular activity, and he basically said no problem, it's not everyone's cup of tea, and he said he respected my desires and he wouldn't push. But he was only nice about it for a few weeks. After that, any time I asked for any sex thing I like, he would refuse and say "why should I ____ for you when you won't ____ for me?" On his birthday or our anniversary, he would ask sweetly for the thing, I would remind him that it made me uncomfortable, and he would then tantrum and refuse to have anything to do with me for the rest of the day. Once in a while, he'd get me a random gift or take me out, then as soon as we got home he'd start trying to guilt me into doing the thing. It got to the point where I would get sad whenever he was nice to me because I knew he was only doing it so he could guilt trip me later, then storm out when I refused to do the thing. He wasn't a very nice person, but I was young and did not see it at the time.

I got really tired of politely reminding him that he has promised to respect my desires. Almost a full year into living together, he decided to tell me that he didn't like "regular sex" and could only get off in this particular way. I was pretty upset, because that meant either he'd lied to me when he said it wasn't a big deal that I didn't want to do the thing, or he was lying to me then because he couldn't live with my preference to not do the thing. Either way, he was not honest, nor did he respect my desires as he had promised too. He was an abusive jerk, and believe it or not, this is some of the least rude stuff he did to me.

If he'd approached it differently, I may have been more willing to experiment. But he did not ask me to experiment, he expected me to provide the full experience with no clue what I was doing and little more than fear attached to the idea. He acted entitled to it, and treated my trepidation as an irritating obstacle. I've since tried with the thing with my loving and patient husband, and I can now confirm that I definitely do not like the thing. I'm glad I learned that with my husband and not my ex, because he would not have cared that I didn't like it and he would have demanded it even more.

TLDR: 1) if a fetish is important to you, be honest with your partner. If they don't want it but you need it, respectfully part, and find a person who can fulfill your needs. 2) if your partner expresses a distaste for your fetish and you can live without that particular thing, respect your partner and do not try to guilt them into doing something they don't like. 3) if your partner insists that you do something that makes you uncomfortable, especially when they emotionally manipulate you or tell you that you "owe them," that is literally sexual abuse and you should get out before it gets worse. 3) if your partner is reluctant to try something, gentle persuasion works a lot better than demands.