The Labour Party was founded to represent the interests of the Labour movement and working people. This is also why it's called the "Labour Party", and not "The Progressive Party" or "The Social Liberal Party".
This is also why it has been a longstanding tradition in the party that social issues are matters of conscience, which is why there have always been a wide variety of views on these issues. This also helps explain why support and opposition to Abortion and Homosexual Law Reform has always been bipartisan.
People who do not understand this are misinformed, insane and are not worth listening to (there are many).
The issue of Progress is that it stands for a radically different agenda to that pushed by the union movement. This is why the union movement has pushed for its expulsion.
GMB's opinion and investigation (.zip):
http://liberalconspiracy.org/images/misc/progress/progress.zip