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While he didn't force himself IN, he continued to have sex with her when she was literally crying and saying quote "Wait please", "It hurts", "Stop!!" and every single one of those statements went unheard until she said Stop the second time from the top of her lungs. Thats why people are calling it rape/SA. Courtney at the very least wanted him not to move/move slower while she adjusted to his size and he didn't which led to her being in a lot of pain. Even though he did eventually stop when he heard her, it doesn't undo the fact that she was intimidated, hurting, and overwhelmed and needed to go slow.
Now, because this is a webtoon, we kind of see both sides. When Courtney said stop the last time it didn't look like he was just lost in the sauce; it looked like he was snapping back into consciousness because he was saying "Huh? What happened?". So we, as the readers, know that Richard wasn't being intentionally malicious by continuing, but at best being overzealous and inconsiderate or at worst being cursed or something(since we've seen the crazy eyes a few times in the story before this, hinting at something being a little off).
TLDR: That scene left a bad taste in people's mouths because it reflects a lot of people's ACTUAL experiences/fears where a partner didn't listen to them and tainted the entire experience. But because this is fiction, we see the grey area because we have both sides of the story(almost) and know that they're probably going to have a conversation, Richard is gonna get his balls stepped on, they're gonna have another go at sex at a later date, and everything will be fine.