It's funny. I'm not usually big on cross-dressing, but this was hot for three important reasons.
1) Knee socks. I have a thing for men with shapely legs in knee socks to begin with. This probably relates back to watching soccer matches as a schoolgirl.
2) Though he's wearing a skirt, you haven't feminized Sirius. Just described his ass in detail, which is a good thing.
3) Cock straining against the panties. That's just plain hot.