None communication people! Damn frustrating. This is what I don't like about a long manga, they always throw in a misunderstanding, so small and irritating, but would drive the MC apart. It doesn't make sense to me that Junpei would have sex with Sagan when he said he was straight. So when he falls in love with Ei, he's now bi and can sleep with any guy? My guess is the author might do that to make them equal, and offsetting each other's mistake. Or maybe Junpei was imagining that he was having sex with Ei?
Though I love the last chapter where they confessed their mistakes to each other. Still I hate Junpei for making his own assumption and his the lie he made, without both their wouldn't be chapter 15 - 19!
Koi to wa Yobenai