i had this stupid fantasy of being in love with someone
i had this stupid fantasy about being in love with someone
(i think you would need a preposition anyway,even without the "about",it would be sth else as "of"in this case.we need this proposition not only to connect but to address the idea of what the previous one is about.)
FYI,i can't guarantee I am comepletely right.Just check it up with someone later.