Alice Munro 
Post and Beam (Storycuts) 

Lorna finds herself strangely attracted to Lionel, a former maths prodigy and student of her husband”s. When family affairs call him away and his absence coincides with the unwelcome visit of her cousin, she grows ill at ease.
Part of the Storycuts series, this short story was previously published in the collection Hateship, Friendship, Courtship, Loveship, Marriage.