There are many times when friends develop traditions they expect to carry on for a lifetime, but these can become complex as they move into long term relationships. If their partner is not interested in a group outing every year, it can cause problems between the two of them. It can also create issues within the group if they make their lack of interest known, so preventing this type of issue can be important when the relationship is first established.
Some couples have found it is best that they spend a few days away from each other every year, so going to a special destination with only friends can become part of their own ritual. They could benefit from this time by being able to really miss each other, but they might understand their partner is willing to forego being with them so they can each spend quality time doing what they want.
Partners who are coerced into going along on a traditional trip might resent it, and showing everyone in the group how they feel makes it less enjoyable for all. While they do have the right to not enjoy their own time, ruining the pleasure of others is a selfish act. It can make their partner less appreciative of them, and it can result in neither of them being invited back the next time the group gets together. Cutting a person off from their friends is a serious offense against their ability to socialize, and it can even be considered hostile behavior.
Some partners might not fully be engaged in all the activities of the group, but a friendly or outgoing attitude can make them acceptable companions at this time. Those who work hard to get along will make their partner happy, and the group will do their best to accommodate them when they recognize how much effort the person is putting into their relationship.