I sometimes find the perfect gaydar in historical romance unbelievable, too, although I guess it might also get tiresome for every single book to spend half its length with both leads moaning, "But he would never love me, for alas, he probably loves ladies!!" But still! A little variety. Allow the characters some confusion. And it's okay if characters' identities don't map perfectly onto sexual identities as we understand them in the 21st century! Really, that's allowed!