Is Nandi (bhakta/vehicle of Mahadeva) married or bachelor? If he is a bachelor, then how and why does he possess knowledge of the Kamasutra? Is it appropriate for bachelors to study scriptures with sex-education?
Nandi is married. The story of Nandi's marriage is described in this excerpt from the Shatarudra Samhita of the Shiva Purana. As discussed in the Shiva Purana excerpt quoted in my answer here, Nandi was the son of the sage Shilada. He engaged in Tapasya for a long time, and then Shiva appeared before him and made him the leader of his Ganas. Nandi describes what happened next:
Then at the command of Shiva, there Vishnu, Brahma and other gods married me with enormous festivities. Suyasha the divine daughter of Marut, who delighted the mind and heart became my spouse. Suyasha received a chatra which was clean like the moon over which the damsels moved flywhisps.
By the way, when it says Suyasha was the daughter of Marut, that's another name for Vayu the wind god.
In any case, I don't know whether the notion of Nandi authoring a Kama Shastra has any basis in Hindu scripture.