it's a convenient plot device to make the wife "bad" in some way or other so the author can justify the affair, but let's face it - the hero is a total failure for not being able to tame the princess, train her to do what he wants and keep her in shape, or failing all that to realize she's not for him at the very start of their relationship.
and on another tangent, we got the convenient cover story about why fmc's armor was skimpy by the mc's choice, but now look at her "subordinate" - what's the excuse this time?