a-baby
Explanation
The Joke
A woman excitedly tells her partner, "Honey, I'm pregnant! We made a baby!" The man responds with equal enthusiasm but frames it in an extremely unusual way: "Oh, wow! It's like you had sex with both of my parents at the same time! Or all four of my grandparents!" The caption reads: "Never marry a geneticist."
The man is technically correct -- the baby carries genetic material from both of his parents (passed through him), and by extension from all four of his grandparents. So in a purely genetic sense, the baby is indeed a product of all of those people's DNA combining with the mother's. But phrasing it as though she "had sex with" his parents and grandparents is a hilariously inappropriate way to describe heredity.
The Humor
The comedy works by taking a scientifically accurate observation about genetics and expressing it in the most socially horrifying way possible. The man is not wrong -- a baby does combine genetic material from multiple generations -- but his framing makes a beautiful moment of pregnancy announcement sound like a description of something deeply disturbing. The caption "Never marry a geneticist" perfectly captures the idea that some people's professional training makes them incapable of engaging with normal human moments in a normal human way.