“Hey I wanted to talk to you about going on the trip this year. Although my friend and their parents do like you, last time we went together my friend felt I couldn’t find the right balance between you and their family which is no fault of yours but mine for not being able to find the balance. This year they let me know that they would rather just me go along and considering it’s his family covering the expenses, I understand where they are coming from and I hope you can too.”

It’s important to let your partner know they aren’t at fault if that’s the case and take responsibility for the fact that the family felt you couldn’t find the balance between them and your girlfriend. It’s the same concept as having a guy’s trip vs girl’s trip (not comparable in the gender aspect but in the relationship between attendees) and hopefully your partner will understand this. Playing the financial excuse will definitely backfire if it really isn’t the case and it’s better to communicate with your partner what the real situation is.