If you're lucky enough to have an understanding partner, and they are brave enough to tackle your fantasies, would there be any reason to stop them?
There could, of course, be loads of reasons. However, one comes to mind: miscommunication. If you've gushed your fantasies, it's very difficult to separate what is in your head (and should stay there), and what part you want to act out. Tiny steps! There's enormous fun to be had on a voyage of discovery. Take a slow steamer, instead of a supersonic jet. You can relax more and perhaps admire the scenery.