I had the same situation happen to us a few years ago - a friend invited only me to her wedding and not DH (who was DF at this time). Thing is, I had only known this girl 1 year more than what my DF had known her! As a result, I cracked it *lol* and only went to the ceremony but not the reception. Given I had just had an engagement party 3 months before and had invited her AND her fiance at the time, I thought it was quite rude of them to not return the same favour.