They NEVER miss a trick.
When Ah waz a lad...Any woman who had a child "out of wedlock" was talked about all over the village, as was the kid (Like he had any say in the matter), and the norm was to get married THEN have the kids.
Since the world turned upside down the NORM appears to be
1. "Have it away" somewhere or other.
2. Decide to cohabit.
3. Have a sprog or three.
4. Contemplate if there's any point in marriage.
5. Do so, if it suits you after a few years' practice, &/or you'd like your little girls to be your Bride's-maids.
Evidently the card-makers have spotted this trend and reacted accordingly.
I did not, however, spot any for "Dad and his bit-on-the-side".