I know asexuality doesn't have a proper definition or explanation, but half of the things you've wrote is bullshit
Asexual basically means you don't get sexually aroused by just looking at people.
Ace people can be sex-positive, which means they can like having sex and do get physically aroused, but many ace people aren't interested in sex at all, some are even repulsed by it.
If a person only feels sexual attraction towards another person after developing a connection/feelings for them, that's demisexual.
And romantic feelings have nothing to do with sexuality, if someone doesn't feel romantic feelings or to a lesser degree, then they are aromantic, which is not the same as asexual.
You can be noth, aromantic and asexual (aro-ace), you can be hypersexual and aromantic, sexuality and romantic feelings aren't the same spectrum. And being hetero/homo/bi/pan also can be combined with ace/demi, because you might have a preference for different genders while still being ace.
I forgot to add- guys who are GAY for just that one guy- the term is ASEXUAL- they develop attachments to a person NOT based on sexual compatibility but emotional, cognitive compatibility- meaning, the attraction is based on anything OTHER than sexuality and gender- so asexuals can switch from gay, bi, hetero, homo depending on WHAT triggers the attraction.
Like Eunsan, they might not feel any sexual, romantic, physical desire for the other person (hence the label "asexual") and some real-life asexuals can have romantic feelings BUT not be able to express them at all, or much worse, be so shy as to shun or avoid sexual and romantic displays of affection.
They do express love and feel love just as deeply as others- but only for that ONE person they feel connected to. Some say it's pure, innocent love- maybe so... but it's hard to say since only 2% of the population on the planet would technically use "asexual" to describe their orientation. What's sad is that they often have break-ups for their inability to be more expressive romantically and sexually.
In other words, Ian has a LOT on his hands when it comes to Eunsan.