The other thing is he put himself in the non-representative "normal" gays and this manga is a bara. The macho hairy guy who is gay is also mispresented a lot by the media and is quite stereotypical.
If we exclude this and what you stated i have to say it was very beautiful.
I have a theory, though I could be very wrong since I'm only speaking as a white queer girl from the UK who only visited Japan once lol, but I think the MC may be referring to the stereotypical misrepresentation of gay men on mainstream Japanese media. These guys - from what I've seen from Japanese entertainers like Hard Gay - are very flamboyant and feminine, flirty and awkwardly in-your-face about their sexuality. The MC in this manga might have referred to differentiating his and his boyfriend's body type and attitude in contrast to what the Japanese audience are exposed to on comedy TV shows, where they're often made fun of for their flamboyantly feminine traits. I've also seen it in popular anime shows like One Piece and One Punch Man where the "comedic gay" wears unflattering makeup despite having stubble and facial hair and speaks very flamboyantly to make the straight men uncomfortable.
Whereas, you're completely right. Bara is also just as stereotypical. It's similiar to the "bear" gay culture in the west, referring to buff muscular gay men. Though this is just a subculture in the LGBT community, this is far from what "normal" gays are when there's such a variety of us. You're absolutely right that there was no need to say that he was "normal", but I'm wondering if it's just a phrased in a way of saying that he's just different from the harmful stereotypical gay men in Japanese media. Like you say, maybe it was just easier to explain to his homophobic parents who have that stereotypical mindset fueled by that harmful media they see on Japanese TV, rather than stereotypical bara which is targeted for gay men.
I'm not an expert though, this is just something I thought about from what I've seen.
This is a very beautiful manga, and a very nice representation of how it's like to be gay in Japan I think.
However I'm not a fan of that difference he puts between him and the more flamboyant/feminine gay men. We will only be really equal and accepted when the more flamboyant, feminine and butch lgbti people will be accepted. It's not by being like "how im normal, unlike them!!" that our status is gonna be better.
I totally understand that this guy would say something like this in order to be accepted by his parents. This is a very human reaction, I think. But it's also important to remember that the more visible lgbti people are the one who are in first lines when it comes to get discriminated and experiencing violences.