Definitely not. I see a couple of guys wear them at the gym and they look totally ridiculous in my opinion. They are just showing off their package I think. All the other guys have no problem working out in regular workout pants, including me, so I don't get it.
For exercise, sure. Tailgating in the cold and need to layer up, sure. Outside of exercise I can't say I've ever seen that. I wouldn't be opposed to guy's wearing leggings as long as it compliments their outfit and they look nice