That nobody in the Foobiverse knows anything about birth control!
I never knew that Michael Patterson was a "surprise." But his younger sister, late-in-life baby April, certainly wasn't planned. And Michael's spawn were both "whoops" events, even though his spouse is a pharmacist who has at least heard of barrier methods. (Liz Patterson may be the product of an early experiment in in vitro fertilization, but don't quote me on that.)
Johnston's depiction of reproduction in modern marriage strikes me as a tad creepy. The husbands are seed-slinging doofuses who don't have a clue that they might actually, well, mate with their mates. The women are either lazy/forgetful or downright manipulative. None of these couples seem to have sat down and actually discussed family planning.
Oh, wait, Granthony browbeat Therese into producing a daughter to bless their spite-filled union. And we all know how well that turned out.