His sexuality was never officially stated anywhere so we can only speculate. However considering Hisoka´s nature I doubt very much that he has a preference between sexes if you know what I mean. Btw he is not married to Illumi. Illumi is weird and he used the word engangement was a way to signify their contract to each other. I wont say any further cuz I dont know if you are caught up with the manga or not.