There are lots of reasons people find each other attractive, and it can often be a process or it can be immediate.
In social psychology, they often talk about people who have commonalities forming social groups. And of course, a concept that will be very familiar to romance authors, is that of proximity. The typical stages are that a person starts at disinterest, and through repeated contact moves to interest, then perhaps adding passion and commitment.
Am I saying characters have to proceed through all these steps in order? Of course not. In fact starting with passion but assuming no commitment is a pretty common order of operations in life and in fiction.
The other thing to consider is that physical attraction is only one available aspect of attraction. Folks on the asexual spectrum may be less motivated by eyes, jawlines, or other physical characteristics. They may be more interested in a person who they have gotten to know in an intimate fashion prior to any sexual behavior.
The background your characters have may affect their awareness of these ranges. They may have felt that the only intimacy that counted was post-coital.
Some of this will be affected by the age of your characters. The internet has made far more information available to characters that grew up with regular access to it. Plus things like fan fic – depending on the fandom – may well have exposed them to a range of information about sexuality and attraction.
Much of this – especially as it feeds into your character’s backstory, may not ever make it onto the page. But all of this should be considered as you build your characters.
Asexual Attraction: Warning – This link includes some discussion of sexual assault. - https://www.goodtherapy.org/blog/50-shades-attraction-understanding-asexual-spectrum-1121184