What is romantic about Samuel Taylor Coleridge's poetry?
First, it is useful to distinguish between the popular 21st century use of "romantic", meaning pertaining to a certain idealized view of sexual relationships, and the literary term "romanticism" which refers to a specific movement in British and European literature in the late 18th and early 19th centuries. A description of Coleridge as a "Romantic" poet refers to his belonging to this literary movement rather than the suitability of his poetry for Valentine's Day cards.
He is typically Romantic in his use of Gothic, exotic, or legendary themes in his work, such as the Orientalism of "Kubla Khan" or the ballad quality of the "Ancient Mariner". As both poet and literary theorist, he shared the typically Romantic concern with the individual creative intellect, and artistic form as expressive rather than conventional. He was interested in art as a striving towards the sublime.