The thing is sexuality is a continuous spectrum (0,1). Using advanced mathematical apparatus we can prove that every single person is bisexual. That's why mathematics is fundamentally gay
Moreover we must consider intersex people. What if their stuff is 20% dick and 80% vagina?
No.
You just say: I don’t wan to have sex with you.
You don’t need to explain yourself, like a wimp, or go into muddy waters of stupid definitions