All of Ayako's worries stem from this misconception: she's amalgamating "correct" and "normal" and "right" with "going with the crowd, doing just what everyone does." This is a horribly toxic and dangerous way of thinking that has historically lead to the damning of homosexuality, polyamory and miscegenation (to name only a few). Ayako has persuaded herself that her feelings are wrong because society would disapprove of them. This is bad and sucks big time.
Very well put. I think there is also a note of puritanism, that things involving sex are somehow intrinsically corrupt in a way other activities aren't (actually that could sort of apply to most of the items on your list). That sex, especially pleasurable sex, is come sort of irrevocable corruption. People, apparently including Ayako, get hung up on the sin thing and lose track of whether or not there are real harms.
As others have put it, situational ethics. Looking at the context of the particular case, not grasping at some universal rules about right and wrong.
Asuka and Ayako enjoy what they are doing and they are both willing participants (Asuka in particular has had a very clear idea of what she wants for a long time... we are probably due for her to explain to her mother how long and how deep her feelings run). In many ways it is very nice that they both have an opportunity to explore their first romantic experience with the person they love and trust most in the world. A bit like lending money or going into business together it only has to harm their family relationship if they let it.
The age thing is more of an issue than the incest. Self knowledge wise, sixteen is worlds better than 12 or 14; it's true 16 year olds still fall a bit short of full adult decision making capacity (whatever that is worth), but Asuka isn't considering international treaties, pregnancy or a facial tattoo... it is just exploring her feelings with a woman she loves and desires... well within teenage decision making capacity.
tbh I hope the question they consider is not "is this true love?", which neither of them is experienced enough to answer, and taken seriously would be a commitment too far for Asuka's age, but just "is this what we want to do, now?"
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