I'm going to ditch my girlfriend on valentine's to go to dinner with another girl
Why would you ditch your girlfriend to take another girl out on Valentines day?
She is really cute and I want to go out with her.
That is a terrible thing for you to do especially on Valentine's day. It is not acceptable to spend time romantically with someone else besides your significant other. If your girlfriend finds out she will probably break up with you.
All is good. She's fine with it and as long as she doesn't find out I'm fine with.
It's really selfish and a betrayal to go behind your romantic partner's back to spend it with someone else. You might be fine with it but she certainly won't be. If you truly care about her then ending the relationship before cheating on her would be the respectable thing to do.