Sure, if it causes intense distress or interpersonal difficulty then just about anything is a mental illness. For every person like that I'm sure there are many

Sure, if it causes intense distress or interpersonal difficulty then just about anything is a mental illness. For every person like that I'm sure there are many for whom it's just a fantasy that excites them to think about but they'd never actually act upon and isn't really a big deal. There are a lot of fantasies (not necessarily sexual ones) similarly enjoyed by mentally healthy people every day.