The determination to drag in romance started with the abortive TV movie (Eight) which was an American co-production, while New Who always also had overseas sales in mind. It seems to me that both American and UK execs believe that you cannot expect US audiences to believe in an alien who doesn't have heterosexual feelings towards a human female. Hence the attempt to make the Doctor half human in one and establishing Rose as a love interest in the very first adventure of t'other.

Before then the Doctor had travelled with a female member of his own species without there being an overt romance (and even with there being a romance between Baker and Ward) between them, except in the minds of ficcers.