You're right about the grooming requiring intent but it also entails manipulation and exploitation. I don't think there was intent, in the way that you think, and I definitely don't think there was manipulation. Tetsuya had condoms because he was the leader of a bike gang, single, and in his early 20s (he likely gets it all the time, I doubt any other person in that position wouldn't have condoms). Also he didn't have lube, he had to use olive oil - he didn't plan to sleep with him then. I think it happened because of the fight beforehand and the rising tension within the gang, making Tora step back a bit (this is where I think Tetsuya started realizing his feelings).
Additionally Tetsuya was 23-24 (someone calculated it), he too was a kid, despite the arbitrary 18-20 year old age of adulthood. Humans don't have fully mature noggins until they are 25. BUT he was older by a decent enough number of years and a highschooler 16-17 years and that puts the situation on him, especially since Tora was already reeling with his feelings about his brother, the gang, and his budding love of Tetsuya.
Now in terms of intent. I really don't think it was there. It struck me in the beginning that Tetsuya saw Tora as a smart ass kid who made the gang a little more fun (Tetsuya even mentioned that he made it less boring). Then a kid who railed up the leader to ride his best by 1. doing his best on his worst subject and actually succeeding 2. questioning his abilities to ride (this leads to shared adrenaline, respect, and companionship). The threshold likely happened (like I mentioned before) when Tora started pulling back and tensions in the gang at an all time high. Now you may argue that Tetsuya having feelings for him and acting favorably to him would be a definition of intent but I will remind you that Tetsuya actually told Tora that he could not ride with him for a while. So if anything, he actually took a step back.
Finally, this is set in Japan (I am not going to say this is right because that is not how I feel). There was (and I think the thinking is shifting) a belief that high schoolers were essentially adults when it comes to relationship. My highschool Japanese friend was actually dating a working corporate man (which really shocked me) and mentioned that it was actually pretty common. You've also probably noticed all the manga out there with highschoolers and adults in relationships. I'm just putting in this point because I think that explains the seme's relaxed attitude when they had sex. If their coupling was a result of grooming, there would be some element of maliciousness behind it (because grooming is malicious and groomers know they are doing something bad) whether it is visible or not. In this story, that is not the case at all.
Also there are no signs of Tora being groomed: no isolation, possessiveness from seme (in fact possessiveness came from Tora), no gifts, no secrecy between them, I would argue the excessive attention since Tora started the whole riding behind him and it was a shared enjoyment (more like friends), there was no fear, and there was definitely no sexual inappropriateness (not including the actual sex scene - this is an underlying theme before the fact when grooming is present).
I agree. While the age gap when they first sleep together makes me uncomfortable (shouldn't be sleeping with anyone with anything below a 2 in the decade place for age), I don't feel like Tetsuya groomed Tora. Yes, he showed him favouritism for just over 1 year, but it was in the presence of others (not isolated), and it was maybe a few hours most days (no way he went out every single night as a highschooler - he passed his test [they even mention not seeing him for a while] and probably got sick at least once). He also didn't ride with Tetsuya for the whole time (he rode with Tomo once he officially joined the gang). And there was no manipulation, that I saw. I do think both were attracted to each other fairly early on, but didn't recognize it. I also think their first time was closer to heat of the moment than planned, for the reasons you state.
For anyone confused let me clear this up once and for all. In chapter 1 p.13, the person spoken to when one of the gang members says "yer 18 yerself" is not Mori. It's not clear who it is but they respond that they're working, and Mori hid the fact that he works from everyone except 2 people (as evidenced by ch.3 pages 4-5). An argument could be made that it's a different job, but ch.4, p.3 sounds pretty conclusive to me. He says that he was a "little younger" than 25 at the time so he must've been 24/23, a far cry from 19 if you ask me. Realistically, what 18 year old is leading a gang like that?
On the question of whether this constitutes grooming or not, how do you guys define grooming? It's any act done to gain a child's trust for the purpose of sleeping with said child. He showed Tora preferential treatment for a whole year. Grooming is often depicted as relentless, but in reality it's very covert and insidious. Some small things you'd never pay attention to can be constituted as grooming, especially acts of kindness. As for whether he was planning on sleeping with him from the start or not I don't think that matters. He did it and it wasn't a heat of the moment thing. He took his sweet time buying condoms and all. So what makes his actions different from grooming? Is it intent? What more could he have done to concretely turn this into grooming?
I understand wanting to enjoy this manga, but please don't try denying the facts. Feel free to create your own headcanon by changing their ages, but I'm sure the mangaka intended the story to be this way. I say that because there's precedent (see Nirameba Koi vol.1, ch.6). I enjoy a few things about this manga, especially the biker gang concept, but let's be honest with ourselves. I respect the people who admit they change the details in their minds more than those frantically trying to deny the facts.