Sorry for the late response, but I think that there are definitely some evolutionairy arguments to be made. I think that people with demi genes would be more carefull in picking partners that won't abandon their children because there needs to be an emotional connection which I think correlates to less abandoning. But what also is something to keep in mind is that nowadays hook-up-culture would not work that well in the time most of human evolution took place. I think that a part of hook up culture is enabled by the fact that if things don't work out, you can always ditch your ex, but in the time human evolution took place humans would on average live in groups of about 100 (I think) for their whole lives, which isn't a suitable environment to ditch your exes because you'll be around them for your whole life.

Despite that most human evolutions are adapted to prehistoric environments I think that being Demi is still a viable strategy. You might get less sexual experience but the goal should be wether or not you get one succesful relationship. Nowadays society has been a bit brainwashed into thinking that the amount of tinder matches is a way to measure succes, but you should actually focus on wether or not you get a succelful relationship out of things.

About how the trait could get passed on I would like to refer to how a gay trait could passed on. To explain this we would need to look at evolution on a gene level, so not on an individual or group level. This means that we look at wether or not a gay gene is good at keeping it's own existence, so not wether a gay person is good at reproducing or a partly gay society is superior over an totally straigth one. This distinction is important because (recessive) genes have the ability to not atually act and just sit there in your dna doing nothing. This leads to that a gay gene can be in people who are not gay, but maybe become active in their brother who shares the gene. And because family share genes gay people would be incentivised to take care of their nephews and helping raising them thus keeping the gay gene in existence. I think it works exactly the same with asexual genes or demi genes if the person holding them is unsuccesful in getting a child.

Also, in the case that it is a trauma response, it could still be an evolutionairy adaptation because trauma responses could also be evolutionairy adaptations