There is sort of an understanding, especially if you are 26 years old.
No matter how you pretty it up, the parents realize that their daughter is introducing them to the guy whom she has picked to screw her. They also know that you know that they know that.
And no matter how they pretty it up, you realize that the girl you are screwing wants you to meet the people that screwed and as a result conceived her.
None of these things are said aloud, however.
Parents accept the facts of life - *****es get ****ed. The romans knew it, the Carthaginians knew it, and now you know it.
There is no reason to be nervous, these things are understood. What is more is that her parents understand as a grown woman she is entitled to choose her own mate. While they have may have influence, it is their obligation to try to like her selection.
Simple tips:
Act like a decent human being. Don't talk like a sailor in front of her parents, especially the first time you meet them.
Dress nice. That doesn't mean try to out dress her parents. You are going to like either a jack-ass or a dumb-ass if you show up to a cookout in khaki pants and a button down where everyone else is wearing t-shirts and shorts. Either way you look like an ass of some sort. Ask your lady friend what the occasion will be, and dress accordingly. Polo shirt and a nice pair of jeans is a safe bet.
Don't be groping their daughter or swapping spit like a horny high school kid in a dark movie theater.
And oh yeah - Bring her mom a gift. Something nice and simple. Have the ol' lady help you pick it out.
Basically, act normal and your going to be fine