What is sexual desire?
Sexual desire is a motivational state and an interest in “sexual objects or activities, or as a wish, or drive to seek out sexual objects or to engage in sexual activities”. Synonyms for sexual desire are libido, sexual attraction and lust. Sexual desire is an aspect of a person's sexuality, which varies significantly from one person to another, and also varies depending on circumstances at a particular time. Not every person experiences sexual desire; those who do not experience it may be labelled asexual.
What is emotion?
Emotions are biological states associated with the nervous system brought on by neurophysiological changes variously associated with thoughts, feelings, behavioural responses, and a degree of pleasure or displeasure. There is currently no scientific consensus on a definition. Emotion is often intertwined with mood, temperament, personality, disposition, creativity and motivation.
So it seems to me that you can consider sexual desire as an emotion. However, the lack of it in other kinds of classification make me question about that. I think sexual desire is common and basic enough to have a consideration in most classifications.