If it was a mother sleeping with her son would you still be so opposed to it?
I agree though, it's not healthy for a an 8 year old to be dependant on sleeping with her parent, when is the right time to end it? She can't sleep in bed with him when she is 11/12/13 and by the time she gets to 16 it would be ridiculous.
But it needs to be phased out slowly, it will only cause her problems if she feels abandoned but it's going to mess her up later in life if it continues like this I believe.
Thing is though, he is her dad and it matters not at all whether you're married or not at the end of the day, he'll always decide what involvement and input you have, he needs to be made to realise he maybe isn't doing the right thing and it's not in her best interests l, by you giving an ultimatum isn't going to make him realise that, domaine research and show him the facts and see what happens!