No, they haven't gone. Don't think I have seen a heavily pregnant woman 'forced' to stand on the tube or a bus in London or anywhere else. On the tube, as well, there are seats by the doors designated for those who have difficulty standing by reason of pregnancy, age or infirmity and anyone not so affected is expected to give those seats up before anyone gives up theirs.
What I am unnerved by is when men who I think are about my age offer me their seat; this has happened quite a lot in recent years.
Reading the article, I get the impression from what 'sources close to' this MP,that she would be annoyed by the suggestion that she sit down. If that is known by those nearby, they wouldn't offer anyway, but quite how it is sexist is not clear; men don't get pregnant, so there can't be any question of inequality, of discriminating against one sex.