After reading some of the comments here, i was expecting something super cliché and boring, but I was surprised in a good way. I liked it overall, I think the rythm was good and the topic was interesting.

Now, I think the main reason I liked it is, being queer myself, I rarely see realistic queer representation in BLs or GLs. Which is fair, since the target is straight women who'd like to enjoy a nice romance, not really actual queer people. The lgbtq+ matters in these romances are often ignored: no homophobia, no identity questionning, no other representation than the main couple... I'm not blaming them, I actually enjoy these romances too.
But after watching athlete I realized how much I actually missed a more realistic perspective on those matters. Not only did the movie talk about Homophobia, it also portrayed accurately the struggle of coming out, of facing your family who has always known you under a certain light and you're not sure they'd understand, of actually understanding the problematics and not just fall in love, and on top of everything, being part of a community.
Although some of the BL/GLs I've seen before do go over the main problematics of being queer, I've rarely (maybe never) seen one that actually depicts the lgbtq+ community. This movie did, and I loved it. The final scene when they talk about priscilla was super touching, and I loved that Kouhei actually gets to learn about the people in that bar and grows close to them rather than just fall in love with a guy and try to understand only him.

Now digging into the main relationship. The open ending didn't bother me. The relationship was pretty unstable from the start, so I wasn't even expecting a happy ending. However, I understand the people saying that it was too much of a mess. The couple raises a lot of topics and problematics: Being a prostitute, cheating, jealousy, being ashamed of their relationship, etc... but even if they reconsciliate at the end, they never actually solve the problems. They never communicate about their feelings, they just blame the other for doing something wrong and that's it. Maybe that's why I don't mind the open ending, because I could see this story expand and have a sequel where Kouhei finds another boyfriend with whom he actually has a healthy relationship, and Yuta would be an important part of his journey but not the finality.
I'm a big fan of "show don't tell", so I also didn't mind them not saying "I love you"s and stuff, but still I was annoyed how they didn't kiss once without having sex right after. Knowing that this story really wasn't about lust, I think that was a pretty weird choice. Also, the first two times were fine but the love making on the beach was just straight up ridiculous. The movement were so weird, the choreography was really bad.
The acting otherwise was ok I guess but I did laugh out loud when that kid started crying in Kouhei's arms at the pool. The acting really ruined the scene there.

To conclude, I think it was a pretty bad BL but it was an good queer story and I'd definitely recommend it to people who look for something a bit more twisted than simple romance.