It seemed poor form to tell the woman that especially at that horrible time in their volatile marriage, Antonin very well could've left his wife and taken his son to start a new family with Ginny. Might have even been glad to do so. She certainly wouldn't have minded. No, it was too cruel to rub that bit of knowledge into her wounds. Antonin wouldn't make the same decision in the present. How she knew that she wasn't sure.