You are, at the very least, thinking about boning his little girl, planning for it, and trying to make it happen as soon as humanly possible. The same little girl he has cherished and protected all her life. This one may seem a blinding flash of the obvious, but it is where the psychological aspects of the father/daughter relationship avert a sharp left turn into Gross Avenue.
As long as his hand stays on that couch everything is nice and PG.
Why You Can't Win
He is programmed to kill you.
Your girlfriend's dad is like the Terminator, and you're Sarah Connor. And the program that has been uploaded into this guy is called reverse sexual imprinting, or the Westermarck Effect, and once you find out what it is, you're going to be damn glad he has it.
Reverse sexual imprinting works like this: if two people are raised in close living quarters during the first few years of life for either or both members, they are somehow desensitized against sexual attraction to each other later in life. In other words, the Westermarck effect is the reason you don't want to bang your sister or your mom, and the reason that your girlfriend's dad doesn't see you as a sexual competitor.
Okay, not "the" only reason.
Instead, he sees you as the guy who is knowing his offspring carnally, and whatever sexual tension a nonrelative would feel after constant interaction with a hot girl in his house is now converted to an overwhelming sense of protection. Protection against you. The enemy. So tread carefully after that inevitable first fight with your sweetie, or you might find yourself on the business end of Daddy's shotgun. He's been itching for the chance to use it.
"You be careful, Ol' Zeke here gets excited when he smells urine."
It doesn't matter if her dad grew up in the same town as you, went to the same schools as you did, or even works at the burger station alongside you. It doesn't matter if you're the valedictorian of an Ivy League school and have a six figure job waiting for you after graduation. You can do the right things, dress the right way and say everything that you think he wants to hear, but your girlfriend's dad is still going to hate you. It's the generation gap at work.
"If only your generation could have a couple of real wars. That'd take care of all this Twitter and Nickleback nonsense."
Now, usually when young dudes hear "generation gap" they think of a lame, befuddled old man who's too square for the new, awesome music and technology. But you have to look at yourself through his eyes. When he looks at you, he's going to see this:
That's Fred, he of the many many YouTube videos where he speeds up his voice and squeaks and squeals and gets millions of hits for it. He even got a movie deal, and everyone over the age of 13 wants to stab their own ears out when they hear his stupid voice and see his stupid face. He's just so stupid and annoying and immature and teenager-y.
To her dad, you are Fred.
Why You Can't Win
While you may want to punch Fred's stupid face...
Really, there's no "may" about it.
...millions of YouTube subscribers love him. Specifically, millions of children. How is it possible? How can they be so stupid and like something that so clearly sucks? You wouldn't know unless you were their age. They wouldn't understand why he's irritating unless they are yours. That gap cannot be bridged.
And that's exactly what your girlfriend's dad is going to struggle with the minute you walk in the door. The things you find funny, the way you talk, the way you wear your clothes and comb your hair aren't going to just be foreign, they're going grate on his nerves. Kind of like when you friend a young relative on Facebook and you end up blocking them for their constant unironic LOL talk and heart-filled statuses.
That's for that little joy, information age.
He will feel about you the way you feel about Fred. Only you now have to imagine Fred humping your little girl.
Young love is sweet. The passion, the excitement, the grand romantic gestures, the precious moments between the two of you. What you may not have noticed is that you have an audience, namely her mom. And she's watching carefully as you shower her daughter with attention, presents and affection, and whether she shows it or not, she's getting jealous.
She smiles while resentment devours her soul.
Guess who's going to bear the brunt of her envy? Your girlfriend's dad.
Why You Can't Win
It's another pheromone thing. Everybody knows about the supposed synchronization of menstrual cycles in women living together. So, in the same way that Mom can subconsciously pick up on her daughter's menstrual cycle, she can also sense those love pheromones that are getting sprayed around like sex-scented Febreze.
You can choose from "Vulva", "Taint Stink" and "Haitian Delight".
But even on a common sense, non-chemical level, it's going to be pretty hard for the lady to not resent your gifts of chocolates and overstuffed teddy bears and 18th century samurai swords that you keep bestowing on her daughter when the best she can hope for is an occasional "atta girl" and love handle pinch from her husband.
On the plus side? All the trashy romance novels you can read.
You can bet your bottom dollar he is going to feel that resentment, and channel it right back to you. This shit has a way of flowing downhill.
When not writing bad comedy, the author gives bad relationship advice on the net.
For scientific good news you didn't know about, check out 6 Romantic Gestures That Backfired Horrifically and for horrifying stories you'll you didn't know about, check out History's 10 Most Terrifying Contraceptives.