Interracial Romance and Multi-racial Anglo-Indian Community
One factor in this might be the advent of a culture that frowned upon interracial marriage starting in the early 19th century. As historians like William Dalrymple have pointed out in books like White Mughals, among earlier British settlers (late 18th/early 19th centuries), British men marrying Indian women was not at all uncommon.
Still, there are a number of narratives published after 1850 that do explore this possibility, including Maud Diver's Lilamani. An interracial romance in the wake of the Indian mutiny is also a subplot of Steel's On The Face of the Waters. In Forster's A Passage to India, the topic is hinted at and an accusation of rape is directed at Aziz by Adela Quested, but the narrative makes clear no sexual assault actually occurs.
A number of other authors also explored the theme, including F. E. Penny (Caste and Creed) and Alice Eustace (A Girl from the Jungle). We hope to add more writings by these authors to the site in the near future.
On the Indian side, one narrative that addresses this, somewhat obliquely, is Rabindranath Tagore's Gora, which features a romantic plot between an Indian woman and a man who is of European descent but understands himself to be a caste Hindu for much of the narrative.
A good scholarly introduction to this topic is, Hsu-Ming Teo's "Romancing the Raj: Interracial Relations in Anglo-Indian Romance Novels." (History of Intellectual Culture, 2004) That article can be found here.