
I think they mean that Natsou been in love with kousuke for a while, he proably though kousuke was straight, so he gave up on confessing. When he learned kousuke was dating a guy every thing changed yes the tactic were wrong, but you need to think Natsou never though Kousuke would fall in love with him, who is a man. So if you where him and the guy you loved turned gay you would start trying more probaly even if that person had a lover, but yes he could have used diffrent tactics.(▰˘◡˘▰)

Ok okay then i understood it correctly, but nevertheless Kosuke never said he was gay he just started to date Sategawa because he fell in love with him & i know it was the perfect timing to try a move but Setagawa & Kosuke don't deserve that, i mean what about Setagawa feelings & Kosuke feelings, all the things they passed & are going to pass because of their love, they deserve each other. I don't like what Natsuo is doing but i don't hate him because being in that position it's horrible & he couldn't help fall in love but when there's a third weel-kun things get more interesting & the main couple come close... But i think it would be better if sensei would make a side story of him with corresponded love (●'◡'●) (sorry if i made a grammar mistake i was writing pretty fast)

Kousuke never said he was straight or gay. It doesn't matter? He only likes one person? We are assuming that just because he likes Setagawa that means he's gay. But no, he simply likes one person. That doesn't mean Natsuo ever had a chance. Jealousy is really ugly.
natsuo has always been the chill guy. it must be hard not falling for kousuke though, he’s a really great guy and they’ve been through so much. he must’ve felt like he got kousuke stolen from him, especially when he “had him or known him first”. there’s that feeling of “i know him better” or “i’ve always been here, so why?”
anyway im excited for his story when he starts loving again after his broken heart.