I honestly don't get your "demisexual" can't also be "heterosexual" comment. As I understood it, "demisexual" and "greysexual" applies to where you are on the spectrum of "shag anyone with a pulse" to "very very picky" (where for me being picky is actually a positive thing - it shows you have good boundaries in place). Heterosexual, homosexual, bisexual applies to the biological sex you are attracted to (and yes, it is biological sex, because gender is an oppressive social construct, and a necessary but not sufficient part of sexual attraction towards someone is them having the right genitals, and there is no such thing as a lady penis, however much the likes of Riley Dennis want to prop up rape culture by guilt-tripping lesbians into having sex with them). "Asexual" strikes me as different again - it's about libido (one can be "demisexual" and have a high libido with one's chosen partner; conversely I've met people with multiple partners who are doing it not out of desire but because some event has left them so traumatised that they can't see sex as anything other than sexual currency to make people like them.)
So it's perfectly possible to be demisexual (want some sort of emotional connection to someone's personality) before wanting to shag them, and also want them to have a particular type of genitals, making you hetero/homsexual, and have a very high libido once you've met that person.
Whenever these conversations come up on here, I'm reminded of a great spoof someone wrote on here about "coming out to your mum as demisexual"... "Yes mum, I know you were looking forward to supporting me through all those rights of passage like getting a bumper pack of condoms, first visit to the STD clinic, and I know how gregarious you are, and you were looking forward to making small talk to a different young man every Saturday and Sunday morning when I brought him downstairs in the morning. But, I'm afraid it's not going to be like that. You see (lip wobble) I'll be taking it slowly, getting to know one bloke, then bringing him home - just that one person, not a whole string of them - every weekend..."
Though I do see in a sense why the current generation has had to invent these labels for what's actual a fairly standard and traditional attitude towards sex. Like young women identifying as trans in vast numbers, it's actually a reaction to our pornified society which now defines "normal" sex as multiple partners, multiple orifices (often at the same time), constantly up to shag any man with a pulse who presents himself. Porn culture has such a stranglehold (sadly often quite literally) on dating behaviour and wider culture that one has to have an "excuse" - "I'm demisexual", "I'm a transman" - to gain "permission" to opt out of pornified behaviour.
And of course the tragic thing is it doesn't work - cf the thread running at the moment about the transman raped by a man who wasn't buying into the myth that this was really a man rather than just another female body to be raped. Not only does it not work, but it strips us of the language to talk about men and women, male and female socialisation, the culturally conditioned power imbalance between the sexes, and the actual root of the problem, which is male violence. We're left just putting sticking plasters over the symptoms.