Christopher said:
The Kirk of the first season was a much more no-nonsense military type than the womanizing rogue his later reputation would suggest. His romantic storylines often involved him resisting women who were throwing themselves at him (Eve in "Mudd's Women," Helen Noel in "Dagger of the Mind,"* Janice and Miri in "Miri"), and when he did seduce women it was a coldblooded tactic to manipulate them (Andrea in "What are Little Girls Made Of," Lenore in "Conscience of the King") -- or else the action of an unleashed part of himself that he usually kept under tight rein (evil Kirk's assault on Rand in "Enemy Within"). Even in "The Naked Time," under the virus's effects, he only lamented about his loneliness rather than going after female crew, and in "This Side of Paradise" it was his unrelenting discipline and hardnosed sense of duty that made him resistant to the spores. We saw a couple of women he'd apparently cared about in the past (Areel Shaw, Ruth), but they seemed to be serious relationships rather than casual flings. And then of course there was Edith Keeler. In that case, Kirk's strict discipline did give way to his passions, but presumably the idea was that Edith was so special that she was the one woman who could break through his shell.
*(Okay, technically Kirk threw himself at Helen in "Dagger," but only after he'd been brainwashed into thinking he was in love with her. Until then, he was quite uneasy with her reminders of their flirtation at the earlier Christmas party. And it was she who very unethically implanted a false memory of them making love after the party.)
Clearly this changed in the later two seasons, where Kirk's reputation for chasing everything in a skirt became more fully established. I wonder how much of that was at Shatner's insistence and how much was at the network's (since the girl-in-every-port Kirk was more in keeping with the standard '60s TV action hero type). But first-season Kirk was a somewhat different character, defined largely by his seriousness and discipline. So it's consistent that his perception of Nancy would've been the most detached and unromanticized.