Apparently after so many cheating Stuart kings, when William III never remarried and he only had one suspected mistress (arguably no mistresses), there was rumor that he preferred men in that way, and this rumor continued all to the time of the French revolution. No 20th century historian takes that "evidence" of not remarrying and not having enough mistresses seriously. No one thought George III swung that way.
Would William of Adelin (hier apparent to Henry I until the White Ship disaster) be rumored to be homosexual if he had no mistresses? I don't think so because we haven't yet had the precedent of multiple generations of Kings who hit on any female that moved (William I and II didn't, while Henry I had... lots of bastards).