Yukino gets waaaaay to much hate. The only thing I disliked about her was that time she Slept with that one guy and that's it. I don't really mind her. She isn't as mature as Setsuko Y'know. She's 18 ( which is the age when teens start doing shit they'll regret later in life ).
I didn't hate her for sleeping with the guy. Her entire conception of her self and her plans for the future were based on her assumption that she was straight. Like, even the fact that she's never really been attracted to men is just explained to her by Japan's cult of female purity, she just assumes that one day she'll meet her future boyfriend and everything will finally click into place.
Sleeping with the guy is a move of desperation, she's not attracted to him. But she's in a really bad place emotionally, and she basically tries to force herself heterosexual... And of course, it doesn't work. It's her lowest moment, but after that she does start to come to terms with herself.
It also kinda demonstrates her changing view of her relationship with Setsuko. When she has sex with the guy iirc she still hasn't really come to terms with them being a couple, in the same way that a man and woman would be a couple. Their relationship is kind of undefined and fleeting - at least to Yukino, who just can't really conceive of two women being a real couple, or of Setsuko being willing to commit in that way. So her sleeping with someone else, especially with a man, is not exactly cheating, because she doesn't know what Setsuko is to her, she doesn't think there's any future there. But later on Yukino does feel guilty over it, because now she thinks of their relationship as real, that they're a couple and that Yukino cheated.