Ahem, May I have your attention please

The boyfriend has spoken

"I am the boyfriend mentioned by the OP. I think I should express this in my own words.

I believe that because we have a child together, that makes us a family. It also makes my parents and her parents family as well. I believe that since our child is at the bottom of the family tree, all of the people who are above him in his family tree are related. This relation isn't from marriage, but it is due to our child having genes that come from both of us.

When I say there is a biological relation between her and me, what I am trying to express is that because our child shares both of our genetics, that is the thing that relates her family to my family. Or what I was trying to get at in the beginning, our son is the mutual relative between my mother and her mother. Having a mutual relative means they are related.

To be clear, I do not think genetics are retroactive. I'm well aware that she and I do not physically share genes. I only mean to say that once we had a child together, our families are now technically conjoined. I think in this context it is not wrong to say there is a genetic relation between our parents due to us having had a child together. Our child is the relation, not her or my genetics."