Halakhah su Ketubbot 7:12

In order to distinguish between the two cases, Tosafot suggest that being an ailonit is an unusually severe defect. If a particular woman is undoubtedly found to be an ailonit, the marriage is considered invalid and no get is required. If the defect is less severe, the marriage may nevertheless be valid, so a get is required. The Rosh (Ketubot 7:10) explains that being an ailonit is an unusually serious defect because the primary reason for marriage is to produce children.