En making all sorts of hasty conclusions and choosing to go down the wrong path is on him, but even Yumi was also in the wrong for trying to skip all the first steps to a relationship, jumping straight into attempted seduction of her first love.

Trying to seduce him with the excuse of a cosplay photoshoot was a poorly thought out gamble that placed her romantic feelings, confidence as a woman and self-esteem on the line. The strategy backfired badly, and the unexpectedly unexcited, borderline disgusted reaction from her first love stung her so much she must've felt so ugly and unwanted then and there, damaging her self-esteem and turning the first love into heartbreak and frustrated hate. "Why did I even like this guy?" is a plausible thought in her mind at that time. Being spurned by someone you like vs someone you're not interested in stings differently.

her private parts looked weird or even ugly, but it was clear he wasn't excited and at that point the seduction was a failure. You can see in her face the moment it all falls apart for her. Her plan hinged on a successful seduction where they'd likely start dating after the deed. They weren't gonna progress any further at that point, so having embarrassed herself, with her feelings badly hurt and womanly confidence damaged by what seemed like a rejection from her first love, she left dropping the sharpest insult she could find.

she really did sleep with all the other guys after that, and the truth may as well be somewhere in the middle. It's not that far of a stretch to say she may have done it with at least one or two other guys, in an attempt to rebound from what seemed like a painful rejection, find a new love and recover some confidence about her body. The painfully naive En then caught wind of this and jumped to the logically extreme conclusion, that she was just an attention-seeker fooling around with all the guys, oblivious to the feelings she held before.

If he knew she was in love with him, and she knew he was a naive hentai addict it might've ended differently. Maybe avoided that poor choice of a gamble altogether. She worked out a plan to capture him assuming he was a simple nice guy who was easy to seduce. He reacted assuming she had no feelings for him and jumped to conclusions she was acting out of malice.