Some people with fetishes try to suppress their urges but ultimately it leads to them being unfulfilled in their sex life with their spouses and seek out risker means to satisfy their fetishes. This can range from being abusive, to cheating, paying cam girls etc but the relationship they have with spouse will be damaged. People also use the words fetish and kink interchangeable, your example of someone who is turned on by blood and knife play has a kink since they are able to engage in sex without it. Blood play is just something that enhances sex for them and they can play make believe, a fetish however can be all consuming to the point someone has to masturbate in a restroom if they saw a death fat in Walmart.
A feeder could possibly date a thin a person since a lot of feeders like the fantasy of making a thin person fat, but it just won't be enough. At some point whatever weight the spouse gains will make the feeder want more. Being a feeder isn't just about liking their spouses being fat, they love the control they have over their fat spouse. The need for control could be mitigated by therapy, but control freaks are notoriously the hardest group to work with.