That’s a really good point about m/m books blazing the trail for mixed-orientation relationships, including asexual/demisexual spectrum relationships. I may have mentioned this before too but I think part of the reason for this is because- until recently- m/m romance existed as an almost counterculture (or at least niche subculture) genre where authors got to play by their own rules instead of by the main genre’s established standards. Now that m/m is becoming more mainstream among romance fiction publishers, I hope that doesn’t mean a flattening out of these nuanced dynamics in favor of stringent corporatized formulas.
As for m/f romance, I have read a couple of HRs with bisexual heroines, but not asexual ones yet. I loved the asexual side character Georgiana and her relationship with the gay duke “Lex” in Cat Sebastian’s A Delicate Deception. This probably won’t happen due to the nature of romance reader expectations, but I would love a sequel that focuses on Georgiana and Lex’s mixed-orientation relationship. True, it would be more of a lavender marriage than a romance in the traditional sense. As an asexual female and a homosexual male, they wouldn’t be falling in love with each other in the traditional romance way. But since they both need each other in a kind of beard marriage- Georgiana because it’s tough being a single Regency female, and Lex because as a duke he needs heirs- it would be interesting to see how their lives would play out. Would Georgiana bring another man into the mix for Lex? What kinds of domestic troubles might Georgiana and Lex encounter? How would they navigate having to be intimate for the sake of producing an heir (hopefully with understanding and kindness despite the mutual awkwardness!)? Maybe this couldn’t be marketed as a capital R-romance for the reasons I mentioned, but it would still be a great companion story to A Delicate Deception, especially if Lex takes a lover and the focus is on that. (Are you reading this Cat Sebastian?)
My experience with f/f is limited, partially because there are fewer options to choose from, but an asexual spectrum f/f would be an interesting pairing as well.