300 is a great movie. Everyone who watches it wants to relate to the Spartans, but they have few outlets to prove their toughness. Standing outside in cold weather without a jacket is one of the few remaining outlets. However, we are, in fact, not Spartans. Based on a particularly subjective cost-benefit analysis, playing and watching sports underdressed in bad weather is a poor method of proving toughness.
There is a cost to both athletes and fans for being underdressed: being uncomfortable. Warmth is on the bottom of Maslow’s hierarchy of needs, right next to sex, food, and sleep. Living without something on the bottom of the pyramid is tough, and it doesn’t make sense for a rational human to deprive himself of a fundamental need by choice. Further, being cold hurts on-field performance and makes one more prone to sickness.
The more interesting side of the analysis is the lack of any benefit for almost everyone that does it. Flexibility in cold weather gear is no longer an issue. If you’re a defensive lineman and playing without sleeves helps you play with a sense of urgency and desire to make the other team go three-and-out on every possession, then go for it, but looking tough to intimidate your opponent is a dubious motive without benefit. However, a few athletes monetize their toughness with endorsements, but endorsements and linemen don't have much of a history. For the fan and non-professional athlete, despite their beliefs of what impresses girls, the benefit is all in personal satisfaction, and it is likely that the cost outweighs this benefit.
For all of the people that don’t like wearing a shirt when it’s cold out, it turns out there is a way they can actually get monetary benefit out of it. They could stop using air conditioning and heating, save hundreds in bills, reduce their carbon footprint, and date someone who cares about the environment. If there is so little personal cost to being cold that they can do it with minimal benefit, they should do it when there actually is a benefit as well.