Spock: It has to do with...biology.
Kirk: What kind of biology?
Spock: Vulcan biology.
Kirk: You mean the biology of Vulcans?
Spock: Biology as in... reproduction?
Kirk: Well, uh... there's no need to be embarrassed about it, Mr. Spock. It happens to the birds and the bees.
Spock: The birds and the bees are not Vulcans, Captain. If they were... if any creature as proudly logical as us...were to have their logic ripped from them...as this time does to us. How do Vulcans choose their mates? Haven't you wondered?
Kirk: I guess the rest of us assume that it's done...quite logically.