Isn't it implied in the 1.1 chapter? In yuzuha's monologue right before we see his twin, 'people who know how smart they are and use it are the worst of the worst; that includes this twin too.' I thought that means he feel really betrayed by his ex & yuzuru and didn't he apologized to yuzuha? What for, then?
Well, i admit it isn't clearly explained that yuzuru and reion was really slept together and maybeee they just flirted a little and didn't get down&dirty but isn't it still wrong to do that? And his opinion about 'people who date but never did genitals meeting are no different than friends' still shit so i still dislike him lol i won't apologize for that
(Another note, and this is so unrelated but i realized that i judged yuzuru too harsh bcs i hold a grudges for other shitty twins from another manga here lol i have to apologize for that)
I admit that i judged him too harsh but i still think it is a shitty move to did a 'test my twin's partner: whether they'll sleep with me if given chance'. Idc if he genuinely thinks that it is a right thing to do to protect his twin's heart from cheating and bad dudes or whatever, something bad he did that he had to apologized for still hurt yuzuha..
Yea that part was kinda hazy, but it wasn't clearly stated still. Anyways as of now I'll just enjoy the story and see what exactly happened between the ex and twin, there are a lot of information missing to judge him as a chara. There might also be a reason for the difference in their value in relationships. Well in the end it's only a manga so.... Let's just enjoy it hahahahaha.

sleeping with someone his bro is dating? Can't he even think that would hurt his twin, he is lucky that yuzuha even want to exist in the same place after that betrayal.
And that 'if he doesn't want to do it (sexual thingy) with you he is no different from friends' opinion is shit. A couple don't have to do sexual things to be considered that they're really dating, right? There are tons of other things to do for lovers to show their attraction and intimacy can't be limited to sexual acts too!