פירוש על קידושין 98:4

In all of these cases, we take the most minimal possible meaning of the word. We should note that this does not necessarily mean that one who, for instance, is able to answer one matter of wisdom is truly considered wise. Rather, the issue is that with betrothal, we need to err on the side of caution. If we consider her betrothed and she is not, the consequences are minimal. But if we consider her not betrothed and she is, and then she marries someone else, her kids are mamzerim.