The one that hypothetically would have existed (or be retconned in) between the two characters previously to make this not a trainwreck. Easy throwaway line during the mentioned upcoming dinner with parents "So when did you fall for Peter?" "Oh, [random incident months before the wedding]. But there was no way I'd let on until I found out he felt the same way!" Done and done.
If you're going to start listening to writers, well, down that path lies only pain.