I don't care what my female colleagues do after work. I do not wish to engage with colleagues at social events, I do not consider my colleagues to be my friends, and my sense of self-worth is not dependent on how they feel about me. In fact, how they feel about me is none of my business. We have a productive, pleasant work relationship and that's all I'm concerned with.
If the op is being bullied at work, she should address this with the company, but as she clearly stated, she accidentally found out about this social event after the fact. Her colleagues did not arrange this dinner in presence, taunting her, making her well aware she wasn't invited.
It might seem perfectly fine to you to deliberately exclude one person from a social event.
Because it is fine, by any definition. You are not required to be friends with work colleagues.
However, if it happens repeatedly it is considered bullying.
That's is not even remotely true. We can all never, ever choose to socialise with any of our work colleagues, and as long as we are not bullying them at work, it is not bullying.