Was this a racist thing to say?

At a party recently, a few friends and I were discussing a recent article about how a foetal cells enter a mammalian mother's brain during pregnancy, and may even be transferred to siblings in future pregnancies. This rabbit trailed into a discussion about how a friend of a friend breeds dogs and was pissed because her son let the golden retriever breeding bitch get pregnant with a lab mutt, and apparently this would affect future pregnancies as well.

Someone commented, "I suppose you could say that once you go lab, you never go back!"

Another replied, "That is not funny, it's RACIST."

I can't see how it is at all... it's about breeding, and is a pun on a cliché phrase at best, but I'd like to know if I'm wrong about this. What do you think?