Currently, when a couple isn't married, lived together, had an annulment, etc., only one of them can be listed on the family tree if they had a child together.
My grandfather was born out of wedlock, and since his parents never really had a romantic relationship, they both aren't able to be put onto the tree, despite him being biologically related to both. Or, at least a preferred parent needs to be chosen. Add on top of that the fact that he was raised by his grandmother and aunt. Only his grandmother can be added as a preferred parent because his grandmother and aunt were obviously never in a relationship.