Casually wearing athletic clothes...

[angus]

I'm wearing gym shorts and a t-shirt.  For the next six months, unless I'm at work, it is my standard uniform.  It may also include underwear and shoes or sunglasses if the situation demands it.

You can probably guess how I voted. 

[angus]

Is there anything uncommon in wearing casually athletic clothes?

No it's very common here.  In fact, I think that's the essential problem for the OP:  It's common.  Unlike those of other OECD countries, our sartorial achievements are not of the high-maintenance variety.  If you go to a casino in Monte Carlo everyone dons tuxedo or an evening gown.  If you go to a casino in Reno everyone has a wife beater and either blue jeans, sweatpants, or gym shorts.  We're pretty laid back here.  Well, except for cathcon and simfan.  Especially simfan, who thinks we should all be dressed like the Earl of Sandwich when we go out in public.