So in the context of Red, White and Royal Blue, the phrase “gay as a maypole” is used humorously by Henry as a lighthearted and affirming way to acknowledge his sexual orientation expressing that he’s proudly gay, in remark to Alex expressing his bisexual identity.
因為maypole這字的解釋a tall pole with long ribbons (= narrow strips of cloth) attached to the top of it, the ends of which people hold as they dance around the pole on the first of May