Is it an anatomical impossibility that I'm not aware of? Is it really that uncomfortable for them?
I'm fed up of men taking up 1.5 seats so the second person has to have one bum cheek off the chair to avoid touching them (or their leg in the aisle if they are also a man). Two women sitting next to each other will generally make themselves as narrow as possible so they can both get their bum on the chair.
I have just sat next to a man who moved his knee in when I sat down but almost instantly moved it back out again so our bare legs were completely touching. I know touching another human is not the end of the world and the seats aren't very big, but surely we would all like to not rub skin with strangers for half an hour. Also, it's kind of the principle of it as nearly all women just don't spread their legs in this situation.
AIBU to think it wouldn't hurt them to sit with thighs together for 30-40 min, and WIBU to ask them to put their legs together so we can both fit? (how could I word this?)