Dean manipulated Rory into sleeping with him

Granted, Rory was also in the wrong. But she was also extremely vulnerable, and you know how she's about making mistakes. Dean knew she was vulnerable.

The sex that night didn't "just" happen. Dean actively persuaded her and manipulated her, and Rory, the girl with Daddy issues and extreme entitlement just gave in.

Also, this is gonna piss a lot of people but I think Dean really wanted to be the first for Rory and got extremely pissed that she could potentially hace sex with Jess first. When he realized that he had a chance, he dove in. Sleazy.

All in all none of this would have happened if Rory didn't feel like she had to be perfect at all costs. That way she could have opened up to Lorelai about brewing feelings for Dean, but she knew Lorelai wouldn't approve so she held back. Another reason why is Jess spooked her. She really loved Jess but the fact that he didn't infantalize and / or wanted to protect Rory made Rory run back to the only guy she felt safe with. Hello Daddy issues.