Maureen wastes no time
While John is obviously not the biological Dad, he is Judy's stepdad.
What I find weird is that Maureen didn't waste any time between the death of Judy's father (Grant Kelly) and John Robinson. I mean, we know Judy is 19 years old on the show, and Penny is what? 16? 17? +9 months incubation.... Maureen wasted no time hooking up and getting remarried, that's for sure.
Plus Grant Kelly was literally LOST in SPACE, so she didn't even know if he was DEAD or ALIVE for crying out loud.