This guy was flirting with my girlfriend and I'm going to lash out at her for texting him.
Was she drunk? What is your reasoning for lashing out of her?
No, she was totally sober and I hate the guy who flirted with her.
You shouldn't lash out at your partner. Try to calmly discuss how it makes you feel when she texts the guy. She might be more receptive of your feelings if you treat her respectfully.
She needs to learn that no guy is interested in her.
It's wrong to lash out at your girlfriend and clearly you're wrong that no one is interested in her. You need to treat her and the conversation about this issue with respect otherwise you might end up making things worse.