That's a great point. Now that Justpeachy13 mentioned it, I'm not sure if he has successfully had sex with his ex-girlfriends...I just assumed he did but then suddenly 2 years ago couldn't which was why he became depressed and unable to write anything. Maybe he never had and when he found out about Terada moving in with her boyfriend 2 years ago, he thought he could have sex with his then girlfriend to move on and realized he couldn't.
All the characters living in the shared house definitely have one or some complex issues going on, it's so fascinating to speculate - I wish we could find out more about their backstory! However, with 2 more episodes left, there's no time to explore unless it gets a second season.
Terada has insecurities about her worth. Ever since the day Hongyo took her virginity and then continues to be indifferent, Terada starts to question her worth. Hongyo also has insecurities and seems to have the inability to process and display emotions normally. Like Justpeachy13 said, Hongyo probably really did like Terada during their school days and when he got teased by Terada, he couldn't control it and he escalated things. Then afterwards, he felt guilty but couldn't express it so he continues to act the same (indifferent) but things were just different afterwards. Terada's avoidance and the lack of communication caused both of them to change/grow for the worse. "We are our own worst enemies" comes to my mind lol. Hase has issues due to his hatred for his mother and Terada has such low self-confidence that she does and follows whatever others say, which to Hase is the opposite feeling he gets from his mother. I also think Hase is trying to get Terada because of Hongyo's reaction to Terada. Kuritani is another mystery. I'm not sure if she's just an odd cookie but it's hard to speculate because we actually don't know much about her background either. Her liking Hongyo and trying to get together with him with her quirky personality is just so funny. And how she handles her romantic rival (Terada) is also entertaining and cute lol.
Next episode would be another turning point to see how Terada and Hongyo would interact with each other after her confession and his reaction lol. Him telling Terada about how cats mate as a way to prepare her for his bite was definitely a surprise. Definitely shows us his "unique" thinking and way of communicating....not sure if he intended for Terada or us audience to read the hidden meaning behind his words and actions or not but it definitely got us wondering and thinking....!
Also Hase! I know this discussion is mostly for Hongyo, but Hase is another interesting case lol.