Paul doesn't like going to weddings. Perhaps his own was an exception, but it's obvious that he's annoyed about this one. He doesn't want to get there 30 minutes early - just to talk about getting there early. He doesn't want to wear a tuxedo - he'll be mistaken for a reception waiter, and for a coat check attender. He declares himself ready to leave, with his tie yet undone - for that is Jamie's job. And he doesn't want to give up his pasta maker - but it hardly matters, for the pasta maker was a gift from the bride and groom. Jamie herself was annoyed she was not chosen as a bridesmaid.
And it doesn't get any easier for Paul at the wedding. He finds himself dancing Allet Cat alone. He approaches the bride's father, and assures him that they've been living together for 18 months, so the marriage will work out fine. But the father wasn't previously aware.
Paul and Jamie are treated as outcasts by the rest of the reception party. Paul can't understand why people keep secrets. Only Jamie admits that she herself has a secret. And Paul relentlessly pushes her into telling him, notwithstanding she begs him to drop it.
The secret is that two weeks before Paul and Jamie had moved in, Jamie had a date with someone who was about to leave for Europe, and evidently Jamie had slept with him. And Jamie chastises Paul for forcing her to disclose it. Paul is upset that it happened. The argument escalates, and moves to the coat check room as Jamie decides to leave. In the coatcheck room, they find Paul's cousin Ira making out with a bridesmaid, Cheryl - a relationship that lasted 45 minutes.
On the way out, they find themselves crossing the dance floor just as the lights dim, the spotlight goes on them, and they become the subject of the toast. The episode never really explains what changed their moods, but Paul and Jamie make up on the dance floor, and finally join the party.