Most Helpful Guy
Yes, it is rude although it is conceivable that it is in oversight. What should be upsetting to you is that if your SO's family is this stupid, you have to worry about the genes and morals they passed along to your SO.
Most Helpful Girl
Depends on the level of agreed upon commitment to each other. Some couples can live together and still be casual, others take that as a sign of progressing towards marriage. It's about two people expressing their needs and expectations and determining, together, how to satisfy each other and make the relationship an enjoyable one. If you feel that you should be invited, then express that and also how you interpret the action, of not being invited. Do you feel less valued? Offended? What? You and your partner need to address the differences in expectations and try to find a solution that satisfies both of you.