Either he left or divorced Mila after Lucas was born or possible before when she was still pregnant or he died. We will never know, I guess it up to us to come up with our stories for the family. It is possible that the father of one or two of them at least already had a family and didn't want to leave them.
In my game, I have the MCCC mod and she is married to one of the sims I created but not playing at the moment. I think she is an elder now, so she probably going to die soon as most of the original sims that were adults or elders at the start of the game have died now. I will probably start a new game once University is out.
