Demisexual as a concept makes sense when you realise it was literally invented on Tumblr as a reaction to what girls were expected to do sexually. Sex positivity, hook up culture, porn, degrading sex acts etc.
When THAT is what popular culture is selling to you, as a teen, suddenly the idea of repackaging normal behaviour (normal as in not wanting to duck someone as soon as you meet them; wanting to build some sort of romantic connection before having sex) as a special marginalised identity makes sense. It’s literally a shield for any young person who doesn’t want to have casual sex. Who can blame them?
It does make sense, as you say for reasons that are quite sad to think about. Pure guess but I would say at least half the dating population (mostly, but not solely) women do not want to necessarily have sex very early on or without knowing someone relatively well, or if they do have a more casual hookup, they don’t really enjoy it. It’s not only within the normal spectrum of human behaviour, it is probably every other person out there.
That said, I do like the joke I saw on Tumbr when demisexuality started to be a “thing,” along the lines of:
“Imagine having to come out as demisexual, must be so tough… you have to say: ‘I need to have a level of intellectual connection and/or know someone quite well to be fully attracted to them and want to have sex… Mum, Dad… I’m so sorry, I just hope you still love me!’”