There is no way around it: In a perfect world, we would all have a little two-seat row to ourselves on the public transportation vehicle of our choice.
On the other hand, I move in circles in which consensus has been reached on the unacceptability of certain techniques for achieving that (highly desirable) state of affairs. (And yes, Octopus Grigori blogged about this first, here.) You are allowed to stand up to avoid sitting next to people; you are allowed to be unhappy when people sit next to you.
You are forbidden to sit on the outside seat unless you suffer from extreme claustrophobia and if you, when asked to move aside, do so with grace and good will. It is only acceptable to put your bag on the seat next to you is if the bus is almost empty. Even then, you need to maintain a constant awareness of the influx of people onto the bus. Really, you're better off with it on your lap.
I know the basic rules, and they're ok, but I still suffer from that inner tension between what I want (two seats to myself) and what I understand is required of me as a good citizen (the willingness not only to share, but to make the sharing relatively painless). And so I brook over those not following the rules as I see them, and I tend to say "Excuse me" in that classic passive-aggressive semi-hostile tone of voice and scoot in next to the person sitting on the outside of an empty seat row.
Which is to say that the first time I saw the elderly man in the row ahead of me slide over to the inner seat as people began to board the bus, I was not unsympathetic. He wanted two seats to himself, but he was willing to move if it looked like it would be necessary. Then as the people filed on and found other seats, he moved back out, tentatively. The bus kept moving, and he kept sliding back and forth. Sometimes he sat in the middle of the two seats, prepared for action either way.
It was all an illustration of our haplessness in the modern world; Ayn Rand would have told him to keep sitting on the outside.
But the bus kept going and it kept happening and I realized that he was only sliding in when the people that might need seats were female. Three guys left boarding the bus, and he was back out, walling off his turf.
It's one of the classic anti-feminist things you hear, that we don't appreciate male courtesy. Which is, at least for me, not really true. I am always delighted when people offer me a seat on the bus; there's something nice about watching people sacrifice for other people, even if the rationale for the sacrifice presupposes the weakness of women. I may not take the seat; I do express my gratitude, pleasure, etc. (It's complicated by inclinations towards chivalry myself -- my urges to stand up, to open doors, to pay for things.)
But I guess I found something creepy about this guy's apparent willingness to sit next to someone only if they were female. (I am forced, reluctantly, to confront the possibility that I am completely misreading his actions -- maybe he had a tic, maybe some other algorithm entirely governed his perpetual motion). Was it that woman didn't strike him as threatening? Was it that he thought only women really deserved easy access to a seat on the bus? Was it that he felt bad for women but not for men? Did he actually deep down hope for some pleasant-looking woman to sit next to him?
(Unrelated anecdote: on a Saturday bus a couple of years ago, a guy sat next to me and I immediately thought he was a creep, and then wasn't sure why I thought that. Until I looked around and realized the bus was almost empty, and he could have had a seat of his own in any number of vacant rows. Then he started talking to me and I could smell the booze on his breath. But it was funny: immediate social knowledge based on a subconscious running tally of empty space on the bus.)