I'm thinking of staying friends with a guy my fiance hates
Why do you still want to be friends with him?
It is fine to me to stay friends with someone who isn't your husband in case things happen in your marriage and you want to date one of those male friends.
You should be able to have friends of either sex but it is understandable that your partner might get jealous. You should talk to them about that so that you can come to an understanding.
I can't do that, he will be upset.
He would also be upset if he found out later that you were talking to someone that they hated. In order to avoid problems in the future that might end your relationship, you should talk about them now.