If they were sleeping together why didn't he know how they had babies? They would have had the contraceptive talk and she would have explained they didn't need to worry about it.. but Kes was not shy. She was completely straightforward about biology and she would not have made some jest about aunty yellow flo needed to show up first, she would have explained it to him in detail. Because she was like that. Science oriented.
But he knew nothing! He had noooo idea.