it is indeed assault
That’s what I said ! Why would they write that
tl;dr: strictly speaking, akiyama and shiba's first sexual encounter is assault to initiate their relationship and the dysfunction/wrongness of that is meant to contrast with the paradoxically wholesome nature of their relationship after that.
rape and sexual assault are frequently used in bl to slam characters together and force a sexual( which then turns emotional) connection between characters who otherwise would not be thinking and feeling about each other that way at all within a heterosexual paradigm. particularly in older series like this one, the idea is that relationships between men are such a drastic break from the heterosexual norm that it's only possible as an act of the unthinkable, either a mindless expression of base sexual urges or calculated, deliberate harm. that's the paradigm a lot of bl is working within, especially the older stuff.
this one gives me mixed feelings because it uses assault to set up the relationship like so many other rape to love stories, but I think it has some nuances that mean I rate it higher than other stories that use the same trope... because it's not quite that trope, now that I think about it some more. mostly because shiba is that "don't save him, he's exactly where he wants to be" meme. so if we're talking legally, it's still forced sexual contact under coercion and duress (he doesn't initiate and he can't leave), but he's into the act and the person he's doing it with even though it's not in circumstances he chose. the people directly involved are willing (akiyama because he's curious, shiba because he likes akiyama), but shiba didn't really have a choice in the matter thanks to akiyama's friends. shiba doesn't care about that after the fact because he got something he wanted.
as to why it's written like that? the whole dog motif and shiba being the cure to akiyama's loneliness, and them both being weird, lonely people in their own ways that end up complementing. akiyama and shiba's sexual, then emotional relationship starts by force under the idea of a joke and a means of humiliation but a real connection comes out of it. akiyama's loneliness and lack of deep connections in life means he's running with a a group of shallow friends (apart from sano tomomi since they've got more history) who think coercion and assault is funny, and then unwittingly drop the companionship he didn't expect to find in his lap.
90% of BL stories include assault, this isn't something new (sadly)
Lwk first chapter ain’t that js sa