Being in a relationship is weird, and there is perhaps nothing weirder about getting serious with someone than the inevitable Meet the Parents moment. And it is brilliantly parodied in the Buzzfeed video "Awkward Moments When You Meet the Parents." Because there are Just. So. Many.
The fact that your significant other's parents love them is absolutely no guarantee that those parents will love you, and the fact that you happen to like their child quite a bit is no guarantee that you will like them even a little. And yet, for the sake of this person (who might well be all that any of you have in common) you all have to try to get along and pretend that nothing is weird at all.
And the whole situation is made even more precarious by the fact that your significant other has also probably told you things about their relationship with their parents at some point, stuff that you have in your head the entire time you're sitting there.
Of course, it's not a lot of fun being the person introducing your girlfriend or boyfriend to your folks for the first time, either. After all, it basically requires you to run interference non-stop the whole time.
Still, the most awkward moment isn't the forced friendliness or the moment that someone inevitably brings up politics or religion. The most awkward moment is what happens once the visit it over.