Family Tree Glitchiness

[Cassie]

At first I thought I'd just messed it up somehow, but this is happened on 3 occasions now so I figured something's up.

Sometimes, when my Sims have a baby, they'll WooHoo together but when the baby is born, it ends up only having a mother and the father not actually being its father. So the child wont have a father, only a mother. The father will instead just be a step-father, and its brother's and sisters end up being half-siblings, which is really annoying if you don't want that kind of set-up.

Has anyone else had this problem? :\