I feel like people are misunderstanding Gotou, its clear in the first chapter that she felt like he only wanted sex when he said they should go back to his place but she did like him. Then of course seeing him with other woman, she got jealous because she liked him as well. Yes she should've confessed sooner but I assume it's not nice when it feels like someone wants you just for sex, maybe thats no the only thing he wanted but it was certainly a big part of it. So I can understand Gotou saying she had a boyfriend. Maybe not the best way to go about it but she did. That's just my take on it. But I'm probably wrong and shes a bitch but there ya go.