What ive said has nothing to do with wether it's a thing you turn off or no that's not what im trying to defend here the way it works is you will always like homosexuality or heterosexuality more what you like more is shat you identify as so saying you are bisexual while yeah you can use it do describe your sexual preferences if someone asks your sexual orientation it's heterosexual or homosexual all im saying m8 and If there wasn't any supporting evidence the teacher wouldn't be teaching it

I think I understand what your teacher and you are trying to say, but I don't think it makes any sense (perhaps you've misinterpreted your teacher's position).

If homosexuality is being 100% into guys and heterosexuality is being 100% in women (from the male pov), then bisexuality is a sexual orientation that involves a varying ratio between the two. If you were 40% into guys and 60% into women then you can't call yourself either homo or straight (since these require 100% either way). This means that you'd be labelled as something else - Bisexual.

Do you believe that labelling a dude who is 40% 49.9% into guys as 'heterosexual' is truly correct? Forgive me if I've misinterpreted your argument entirely and this isn't what you were suggesting.
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edit:
You've also made a distinction between sexual preference and sexual orientation (you support bisexuality as a sexual preference but not as a sexual orientation). Can you elaborate on the difference between the two? As far as I'm aware, 'preference' and 'orientation' are just synonyms.