4You are a Usurper
You could be as menacing as a McLovin/Urkel love child, sadistically named McLurkel. You could literally be a girl, it doesn't matter. When it comes to dating a man's daughter, you are first and foremost, a threat. And what you're threatening is his status as the alpha male, so you better take this shit seriously.
He will bite your face off.
Why You Can't Win
He is hardwired to see you as the enemy of his kingdom.
No matter how pathetically wimpish a man may be, he is always the alpha male of his own home and of his own children. For years, this guy has been the dominant man in his daughter's life. Suddenly, his begotten is ignoring him and slinking around with another man.
"Look at all those lemons. What's his game?"
And not even a real man, but some young kid, still wet behind the ears. One who, in the natural order of things, would easily be scared off with a simple growl, but now gets to lurk around and hold court with the king's little princess.
You're lucky he doesn't behead you in the town square as an example to the other horny young males.
On a similar note...
3He Instinctively Wants to Protect Her
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."