His failure in trying to seduce her showed he was mistaken. She was a mother of two.
Literal
Her seduce try-and, failure [at-で], he [subject-が] wrong-guess doing known. She [topic-は] two children mother was [because-から].
誘惑しようとして means 'trying to seduce.' 失敗したことで means 'by failing.' 見当違い means 'barking up the wrong tree.' A wryly humorous anecdote revealing the man's misjudgment.