While William III of the Netherlands was known for his voracious heterosexual (as far as we know) appetites, his father, William II, was notoriously equally interested in young men. That said, you are likely referring to the William III who was stadtholder in the 17th century, Prince of Orange, and king of England, Scotland, and Ireland, whose intense friendships with attractive young men, and relative lack of interest in women, did fuel pervasive and recurring rumors, which he angrily denied, and which he suggested were politically motivated innuendo (perhaps warranted, as pamphlets on the matter were widely circulated). There is no clear evidence, but the rumors were indeed historically relevant.