Continuity mistake: Pepper states in this episode that he doesn't have any kids, but in 4-14 "A Slight at the Opera" he talks about a son he has who is a Navy Seal.
Continuity mistake: Whilst Clare and her new 'best friend' are talking to the flight attendant, the two glasses change from empty to full in the next shot.
Answer: Mitch is upset that his dad doesn't seem to support his getting married. When they talk about Jay's friends not coming to the wedding, Jay makes it seem like Mitch wouldn't want them to come anyways. Mitch sees it as it's actually that Jay doesn't want to invite his friends to his gay son's wedding because he's embarrassed and uncomfortable about having a gay son. Essentially Mitch sees it as if he was marrying a woman, Jay would be willing so spend money on the wedding, host it at the club, and invite all his friends to it.
Bishop73