Maybe it was from a Roddenberry interview, but someone or other once said that McCoy was Freudian Emotions (id? unsure of term) and Spock was Brain and Kirk balanced the two.

Kirk wanted desperately to contribute to the gene pool, but outside of a miscarriage suffered by his wife in one ep, he failed until the second movie ST. I think I prefer Star Wars Jedi rituals stories, where they have to have sex and it's do it and go.