Your GF’a parents are just assholes. My GFs Dad and his family made me sit aside for many years while they took family photos and whatnot. They generally just accepted me as someone who’d be there but not a participant in the family stuff.
His reason, and they told me many times, was we are young and in college. You know those relationships just don’t last long so you’ll probably not be together for long. Going on 11 years.
That side of the family are assholes too.