Everything that makes "news" these days is about context.
What Bill S. said (While I, as a guy, found the T-shirt to be humorous, I can imagine that some folks might well find it to be inappropriate for a family environment.) is the absolute truth. In that context, I also would consider the T-shirt offensive. But then, I come from the day when men ALWAYS wore a tie and jacket on an airline.
FWIW, the carriage contract (what you agree to when you purchase a ticket) allows the airlane to refuse carriage if your clothing is lewd or offensive. Why must we now go through the futile effort of defining in specific terms what constitutes lewd and offensive? Will this definition apply to bus and plane or only to plane? Will it apply to commercial flights or charter flights or GA flights or all of them?
I consider myself a conservative, but I always try to see things in context. In the context, I think SWA was correct, Just as the other recent events may well have been.