I am not wanting to attend my girlfriend's son's birthday because I had plans. It would be awkward to do this.
Why don't you want to attend the party?
I don't want to attend his birthday because we had an argument about an important night and I don't feel right around her and her family.
Your personal issues with the family doesn't have anything to do with the boy's birthday. You should be supportive and put aside differences for someone's special day.
Fuck that. I'm a different person and I won't judge anyone's actions.
I can understand how it might feel awkward to be around her family after a fight, but you shouldn't take it out on the boy. You should always be there for family birthdays. See if you can put aside your feelings and make his birthday special.