Let's suppose for a moment that UGG had paid Tom Brady barges of cash, not to endorse their boots, but to, say, endorse the punching of kittens. Would that affect your conclusion that there was nothing manly about punching kittens? Would you decide that punching kittens was actually a stylish thing to do, despite the mountains and foothills and escarpments and mesas of evidence to the contrary?
The standard UGG boot, considered alone, is a floppy, puffy, "but it's so comfortable" middle finger jabbed in the eye of the proposition that we ought to get a little bloody bit dressed up to go out in public to see people. Why do we let girls get away with it? It's complicated. The short answer is that many of them are better at navigating the "My feet are dressed casually, yet I am still demonstrating that I have a modicum of respect for you" line, by dressing up other parts of their outfit, than many men are. Another answer is, "Sometimes, they pair them with a cute skirt and leggings." The point is, Tom Brady can do it because nobody cares about the clothes he's wearing unless they're a combination of Nylon, lycra, jock strap, and pads, and it's Sunday.
You and I, gentle reader? We are held to a higher standard, every day of the week, and for that, I am thankful.