When travelling I always wear chinos/slacks and a collared shirt - usually because I am on my way to/coming home from work. Having said that I don't really have an issue with other travellers wearing whatever they deem comfortable. Personally, I think too many people are worried about what others do/say/think/how they dress. Unless the clothes are so skimpy that you are on the verge of seeing genitalia then why get worked up about something you can't change anyway? Just don't look at them!
I would argue that just because things have been "accepted by society for a long, long time" it doesn't necessarily make them right or trump any other opinions - cf. slavery, racism, homophobia...need I go on. These too were "standards" by which others were judged.
The only things I find offensive when flying are bad manners (ie. not removing headphones whilst being spoken to by an FA), bad personal hygiene (a shower and a spray of deodorant are always a must), bad table manners (the amount of people I've seen licking out meal trays and eating spilled food is astonishing) and...oversized passengers not booking themselves a second seat and intruding on my personal space. I don't have anything against larger people, but when the space I have paid for is reduced then I get annoyed.
Oh...and intolerance...that offends me