He knows that kouta likes him but still says stuff like "i like girls" and stuffs, and that'll slap. idc if it's to make kouta confess or something but that just makes kouta's confident lower and be like, hopeless. watching your bestfriend/someone you like hanging out and go out with girls all your life will just make you hopeless and now that haruto and kouta's in a relationship i get why kouta's like that and has a lot of insecurities. haruto doesn't give enough confirmation and is just being passive, he does say no but once kouta says sure he just go and doesn't talk about it with kouta so kouta's left confused. while kouta is a pushover and doesn't expect something, they're both passive and that's the problem.
if you're gonna defend haruto, go. but my opinion about him will stay like this until they give confirmation to each other and talk it out. they're both at fault, yes.. but I'm still blaming haruto, he made himself like "everybody's man" and kouta's like a bridge/messenger/delivery man or something to him and to others, its always been like that.
he's still deku, it's just that.. how he perceive the world around him change.