Thanks for the help. I managed to get the achievement for all five girls pregnant in the same loop, but I still can't select Lin pregnant from the corruption menu.
This is actually not true. Like everyone else, you need to find her Corruption MAX event. *SPOILERS*
It's on Monday. You need Lisa at Corruption 5, the mood has to be 12+, and you need to have had the "Quiet Lin" and "Quiet Lin 2" conversations with her on Friday and Sunday. Both times, ask "Are you okay?" During the "New Beginnings" conversation, choose "Sequel Time."