well if this is your definition then we all are committing this sin when watching movies. movies is all fantasy, denoted as hyper reality. (greater than your life)
i think i read an ayat (i will try to locate it, but i wont gonna promise) that as long as it is a thinking its not a gunnah, let alone gunnah-e-kabeera.
but as i said ask constantly for forgiveness and ask ALLAH SWT to get rid of u from these thoughts.
ps. if you have good niat you do get its Sawab, where as implementing on such thoughts are more rewarding. but ALLAH SWT is merciful, and i think he wont punish anyone for any bad thoughts. Be Optimistic! :k: