If your significant other tells you they don't want you jerking off by yourself, run. Especially if they say they don't want you to ever fantasize about anybody other than them. If they want to police your thoughts, get out. Nobody should be in that kind of controlling relationship. Be open and honest with each other. You're both humans. You'll both continue to have fantasies. Jerking off with someone else in your mind, or even looking at porn, is not cheating.