It was logical, if Spock really thought about it, that he and the captain would enter into a sexual relationship. The level of trust and communication required between First Officer and Captain was exponential, and not unlike that of married or committed couples. Almost-telepathic communication between the two seats of command was required, and it only makes sense that if the communication was to actually become telepathic, it would increase efficiency and coordination in the field. And, if Spock were to feel particularly daring, factoring in his human side, it would also be logical to enter into a relationship with someone who could put his mind at rest when sleep was needed and calm the very rare torrent of emotions. All of this, of course, was based on logic, and that his relationship with the captain was logical, and had absolutely nothing to do with the fact that Spock was head over heels in love with the askance human. And it had absolutely nothing to do with the fact that the sex was fantastic.
Hahaha. Well. This is a little thing I cooked up in Math class, and thought it was cute. My spin on how Spock would convince himself that his and Kirk's relationship (coughfuckinglikebunniscough) is logical.
Hope you enjoyed!