Sentence:  Sherlock realizes that John, despite being an <WSD>alpha</WSD>, sees and loves Sherlock for who he is, “a brilliant, mad, nutter of a man,” instead of only wanting him for his reproductive functions, his ability to become pregnant. 
Meaning of the word marked with <WSD> tags:  In omegaverse fan fiction, a person of a sexually-dominant (and sometimes secondary) gender/sex that is driven by biology, magic, or other means to bond with an omega, with males of this type often having canine-like genitalia.