Firstly, my reaction is that this is daft. But not for the reasons you'd expect.
I feel that no-one should have a family unless they can support them. This may put me in the minority, but there you go.
BUT - the fact is that state benefits exist. So do you draw the line and say "your values don't match mine, therefore your family cannot use these instruments of the state"?
Of course you don't . Therefore, you allow everyone - regardless of creed, political view, etc etc - to take part.
If you feel this is unfair, you only have to take on multiple partners yourself and get your money's worth - no-one's stopping you.