Sentence:  That a writer who <WSD>is</WSD> gay cannot be serious is a common professional illusion , sedulously fostered by all too many academics who mistakenly believe that their frivolous efforts should be taken seriously because they are expressed with that dreary solemnity which is the only mode of expression their authors are capable of . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)