Really? Kunihiko was described as being flashy in Asatte no Kiss. He always struck me as the playboy type. I did think it was weird of him to say he always loved Kusaka cuz he himself also seemed pretty confused about it earlier on. Like when talking to Yuu who said his type changed and he's like "did I even have one." So yeah if anything I take issue with that line, but always could mean "for a long time now" rather than "always," and he just never tried anything cuz based on Kusaka's body language and responses he could tell he wasn't gonna make headway with him. He also said he panicked once Kusaka expressed interest in getting a girlfriend, so that's probably either what sparked his realization he was in love or what caused him to take action. Like dude clearly had affection for Kusaka, even if he wasn't sure what it was. And he's allowed to sleep/date others if he's not in a relationship with him. That's my take anyway.
Okay Akihisa called him a "party goer" rather than "flashy" but the meaning's generally the same. While party goer doesn't MEAN playboy, And just cuz you're one doesn't mean the other, but the description is general enough that playboy can fall under there. Since ppl assume that people who like to party also like having fun with people like that. Not always true ofc, but the two descriptions go along together often.
Agreeing with everything you've said
When I first saw Kunihiko-san at Asatte no Kiss, I really want to see more of him. I mean he is really a notable character back then even if he was only shown for a little bit, he left an impact to me I guess. But somehow I was a little bit disappointed on how sensei carried out their relationship here. It was really unnecessary to make a "playboy persona" out of Kunihiko-san, then he goes on and says that he always loved Kusaka-san and yet he can sleep with anyone he catches his eyes on.