It is a civil rights issue. Your religious beliefs do not hold any value on a public bus but it is the civil rights of any citizen to be able to sit wherever they choose. Also, THERE IS NOTHING IN HALACHA THAT SEPARATES WOMEN FROM MEN ON A BUS OR ANY OTHER SPACE. It's called minhag and your minhag in fine in your home - not on a bus.
One student killed, three wounded at Arizona campus during fight (a)