The problem: They would never kiss of their own accord at this point in their relationship.
The solution: Make them kiss. Typically this is commanded by family members, but anyone will do.
This kiss begins as a way to appease an audience, but quickly becomes more to the persons doing the kissing. They realize this, get all awkward and pull away after a meaningful shared look.
I love this.
The best examples I can think of:
"The Proposal" Sandra Bullock and Ryan Reynolds, because you can't say no to Betty White
"While You Were Sleeping" Sandra Bullock (again) this time with Bill Pullman
(this isn't the image, couldn't find the one I wanted - they get suckered by family AND mistletoe!)
while we're on the subject of holidays...
"The Cutting Edge" D.B. Sweeney and Moira Kelly fall prey to New Year's Eve celebrations
Practicing kissing with the "wrong" (i.e. right) person Drew Barrymore and Adam Sandler
and most recently and sparking this post:
the always adorable Amy Adams and Matthew Goode forced into a PDA at a BandB dinner table
I believe the forced kiss is essential to the opposites attract plot, and can certainly help move things along when the characters do like each other. Ultimately the viewer feels the same as the audience in the films - we want to see a kiss too! I just love it.