Hey man, to the old and disabled, I would offer my seat.
They didn't choose to be old. Neither did they choose to be born (or become) disabled.
For the pregnant, it's a totally different ballgame. They made the decision to become pregnant.
U ever had children? U'd let your own wife suffer such discomfort if she was pregnant... I would have no problem fighting to the death for the health of my wife and unborn child. But then again, being 6' 8" and 120kg I've never had problems 'pursuading' the disrespectful to offer their seat for my pregnant wife.
in short though yours is a perfect example. It's the way you've been raised and I'm sure your attitude won't change (until you have kids). Respect and common courtesy should be offered to anyone. It's about moral fibre and strength. Using the 'it's a choice' argument is just a cop out for poor upbringing and low morals. Sorry for telling it straight up. Like I said before, even the most 3rd world countries have got this right. It's just a handful of people that don't understand - they know no better.