premarital father

A male in a relationship with someone who has children from a previous relationship, but you are not married yet.

I'm practically his father, but his mother and I are not married yet, so I'm his premarital father.

premarital mother

A female in a relationship with someone who has children from a previous relationship, but you are not married yet.

What is your relationship to the boy playing baseball? I'm his premarital mother.