I think a division based on the apparent sex of the object isn't really arbitrary. The main purpose of a sexuality descriptor word is to, as succinctly as possible, narrow down the range of people with which an individual is sexually compatible. Biological sex is one of the major ways that humans classify each other and prototypical sexual characteristics are what most of sexual attraction is based on. Describing a man as heterosexual serves as a pretty good indicator of the general kinds of characteristics he finds sexually attractive (although obviously there are individual variations). Other divisions like "demisexual/not demisexual" or "furry/not furry" exist that partially or even entirely describe some people's sexual interests, but it makes the language pretty inefficient to have to include all of these given the small size of the population for which they are useful descriptors.
Well in theory, heterosexuality or homosexuality does not even purport to limit one's sexual desires to members of the human species, again, as far as I know, only gender. I'm not trying to suggest something nefarious about either, I'm well aware most people aren't making that assumption, but there's not a lot succinct about it, to my mind.
Not to mention a person's sexual history or how extensive it is in different directions is I would think a strong qualifier for how people view compatibility. Obviously, gender is also one, but I still feel that information is constrained and limited in ways that suggest the overarching direction. Furry/not furry is pretty limited, that's like a box check, so I don't think that's fair to compare to demisexuality.
Again, I don't mean any of these comments to interfere with the ways people use heterosexuality or homosexuality as positive identifiers, that's fine, I'm just saying these were terms we were, once upon a time, given, and the division was arbitrary, and that demisexuality as an arbitrary division is pretty interesting to me.
It could just be my personal bias, I've grown increasingly fascinated with people's sexual histories and proclivities moreso than just their male/female preference. The fact that some people have had 50 partners in one day and someone else in 10 years 0 is kind of amazing to me.
Edit: I think it's only because something like "heterosexuality" tends to have a lot of built in assumptions, like, I would say people generally assume heterosexuality means monogamous, faithful, one male, one female, and a lot of other things, that terms like demisexual appear unnecessary. I'm not sure those assumptions are inherently part of the definition though.