why do we automatically assume being heterosexual is the default sexuality?
Well there's a good statistical argument for starters

IMO
heteronormativity is mostly at play here. That is, heterosexuality is "ideal" and most common. And I think the Ancient Greeks were no different to be honest - sure they were less rigid in their conceptualisations regarding permissible sexual activity, but they still held 'man + woman' as the highest ideal in sex, sexuality and relationships. Lovers were tolerated, but if a nobleman professed his undying love for another male in a public arena, this was diverging from the ideal, high status position of heterosexuality.
Furthermore, at various times, certain sex acts - particularly between people of the same sex - had certain status attached to them. i.e. it was "ok" for a nobleman to have sex with another man so long as he only performed particular sex acts and the person was of a lower class to them (a slave would be ideal...).
sexuality simply did not exist as a concept
True, but relatively strict normative ideas about what was appropriate and what wasn't were still around.
why do we assume that heterosexual and homosexual and everything else are the best way to classify people?
You could ask the same question about gender and the answer would be very similar - it's easier because
most people are either one or the other.