Here's the lyric: If I pour your cup, that is friendship If I add your milk, that is manners If I stop there, claiming ignorance of taste, That is tea But if I measure the sugar To satisfy your expectant tongue Then that is love, But if I measure the sugar To satisfy your expectant tongue Then that is love, Sitting untouched and growing cold. 'Claiming ignorance of taste' here seems to mean 'claiming I don't know how you like your tea'. In other words, if he stopped 'there', he would be leaving her to add the 'sugar' herself.
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