Dear big dude on the train: when I politely made some extra room for you to sit down, it was not an invitation to throw yourself into the seat and sprawl out your legs so wide that I'm shocked you didn't pull your groin.
I sincerely doubt that you have anything down there that warrants that kind of crotch space, so in the future, please don't look all put out when I "accidentally" use my knee to shove yours back together. Thank you.