"And Dad, I swear if you threaten this one, no grandchildren for you whatsoever."
"Nice bluff there, sweet cheeks. Next time load the weapon you're pointing at my head, it's a little more intimidating."
Implied: While Alice is getting ready, Tom takes Bob aside to have a little chat with him. The conversation is not shown, but by the time Alice is finished getting ready, Bob reappears looking very spooked.
Tom scares away all of Alice's potential suitors, which causes her to fear the very concept of dating. This, in turn, causes her to die bitter and alone.
Tom's very concerned for his daughter's well-being; this just means that the Affably EvilJerk Jock Charles is able to get into his good graces (and into Alice's pants) while the Jerk with a Heart of Gold Bob is left out in the cold.
Bob is outraged at being threatened. When Alice comes back proceeds to bluntly tell her that the date is over before it's even begun because he doesn't appreciate being threatened by people like her father. He then storms out and leaves for good, leaving Tom with egg on his face as Alice proceeds to yell at him for ruining her date.
Bob is so terrified by Tom's threats that he swears off all further thoughts of dating Alice. By the time Tom decides that Bob is worthy of dating Alice, it's too late.
Tom scares away all of Alice's potential suitors...except one. Through his dogged determination, he's able to eventually win Tom over, which causes them to have a lifelong friendship.
Due to his concern however, Tom gradually gets suspicious of Charles when he seems too nice, and runs a check on him. Soon afterwards, Charles gets his ass kicked, and Tom ends up finding Bob is a much nicer person then he gave him credit for soon after.
Played For Laughs:
Alice has been married to Bob for about 10 years - and they have kids, but Tom still calls him every day to threaten him with the "she's my little girl" speech.
Tom is a scrawny, weak, pitiful example of a man, who makes the "she's my little girl" threat to Bob, the star linebacker on the Trope High football team.
Played For Drama:
Tom is a working dad and is often away from home - he, in his mind, views that threatening his daughter's potential suitors is a way to compensate. Alice disagrees.
Bob took Tom's threat so seriously he went to the police and pressed charges on Tom's threats.
Listen son, you seem like a smart kid. That's my baby girl, and if she gets hurt, no one's gonna miss you. I've got a shotgun and a shovel, and I'll bury you myself. So make sure you're back at Overprotective Dad by a quarter to ten and we won't have any problems, OK?