This is an interesting idea, and I'd certainly support it.
I'm not entirely sure I fully understand the nuances of the asexual spectrum of sexualities, but I do have an idea for how a Demi-sexual (or even Asexual/Grey-Asexual) LI could be handled in a game like Dragon Age:
How about a LI who initially refutes the PC's advances/flirts - perhaps even outright telling the PC that they aren't attracted to them.
But then, as the game progresses and the PC interacts more with the LI and gains their respect and approval, the LI suddenly starts responding. Perhaps flirting doesn't get the PC anywhere, but little hints at attraction begin to crop up in their everyday interactions.
The subject of attraction is then breached one more (including whether the LI wishes to engage or not engage in sexual relations) and the LI admits to having developed a bond and an attraction to the PC. The PC then has the option to turn down the LI if they aren't interested, or express their interest in a relationship with them and have the romance begin in earnest.
Now, one big issue I can already see with this scenario is that it might inadvertently reinforce the idea that the LI was interested in the PC all along and was 'just playing hard-to-get' or 'saying no when they meant yes', which would be very problematic.
I wonder if you could avoid (or at least minimise) this problem by ensuring the LI states, in no uncertain terms, that they were only attracted *after* forming an emotional bond with the PC. Perhaps they could also mention that this is the first time its ever happened to them, too, to highlight the fact that this is a genuine formation of an attraction and not simply the LI playing games.