Conan Doyle seems to have had little need for romance. Mary Morstan is introduced in The Sign of Four as a romantic interest for John, and she is mentioned here and there after that until she seems to have died. She does not seem to play an important role except in this first appearance.
It’s pretty clear that despite his desire to remain professional, John Watson is quite smitten with Mary Morstan. Even before he admits he likes her, it is evident from his description.
After the angelic fashion of women, she had borne trouble with a calm face as long as there was someone weaker than herself to support. (ch 7)
Watson is committed to helping Miss Morstan with her problem, even if it means he probably cannot get to know her better because she will be rich and above his station (very important in Victorian England).
While there was a chance of recovering it I was ready to devote my life to the one object. True, if I found it, it would probably put her forever beyond my reach. (ch 8)
John does propose, and Holmes’s reaction is to groan and say, “I feared as much.” (ch 12). For Holmes, “love is an emotional thing” and he cannot let it interfere with his reasoning. Watson seems to have no such problem, and when she dies later he is deeply affected by it. Doyle makes little notice of it. Perhaps he agrees with Holmes when it comes to women.