Girls: Is it wrong to accidentally see up a girls' dress or skirt, and WANT to fantasize about it later?

Recently, I accidentally passed my eyes over a clear glimpse of orange underwear of a woman in her 20's I'd say. She was sitting down with a younger sister (or friend), across the way, in a waiting room of my doctor's office (maybe 10-12 feet away). And I seriously accidentally caught the glimpse (I did NOT try to see her underwear), when she switched her leg crossing, in order to get more comfortable or whatever. But when I got home, I have fantasized multiple times about it, and also "rewound" my memory of the glimpse in my mind to enjoy it. Is this wrong, though, to enjoy seeing up her skirt later on in my mind?