Boys' Night - S2-E18
Other mistake: In the opening scene, the Dunphys are playing ball in their backyard when Claire knocks it into their crotchety old neighbour's yard. They all head in, and as Phil explains to Luke that it's because it's their only ball, another ball is in the grass visible over Luke's shoulder.
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