If anyone asks, I won't hesitate to do it. It's not worth the stress or causing a scene on the way to work. Is it that big a deal either way? If it happens consistently with the same person who seems physically able, then I would ask what is wrong.
Regarding girls, etc. I know a few people who would take offense to being offered seats but if they ask, I'd do it and always would for friends or family.
And seriously, come on CP. We really have it bad don't we? The biggest issue we have to suffer is getting to sit on the train or not? Think of the children in Africa! Africa!
