I always assumed it was a sanitary thing- doesn't really make sense when you're sticking a fork in your mouth though . . .

I never use a straw except at fast food restaurants- and there it makes sense because I'm usually on the go (but I hardly ever eat fast food anyway). I've actually talked about this with folks at lunch and every single one of them prefers to use a straw- I'm the weird one for not wanting to use one. I never asked why though.