Why was Henry Esmond’s marriage to Lady Castlewood controversial?

In William Makepeace Thackeray’s historical novel The History of Henry Esmond, Esq., the marriage between the protagonist, Henry Esmond, and Lady Castlewood was considered controversial primarily due to their established relationship dynamic.

Having been orphaned as a child, Henry was taken in and befriended by Lord and Lady Castlewood. Throughout his youth, he viewed and adored Lady Castlewood as a mother figure. Consequently, his eventual marriage to her in his maturity was perceived by many contemporary readers as deeply unsettling or inappropriate. The author George Eliot famously described the work as the most uncomfortable book one could imagine, reflecting the discomfort surrounding this central plot development.1