Okay, to start, we've got a decent if overused basic concept: straight-laced boyfriend/girlfriend/fiancé/whatever goes to meet significant other's kooky/crazy/dysfunctional family, and humor (assumedly) ensues. Unfortunately, not here, for the most part anyway, basically because the writer (also the director in this case) has no clue how to structure a comedy (or even knows what's funny, for that matter).
First off, Jaleel White's character spends pretty much the entire film either being a) utterly inept, and/or b)obnoxious and annoying, not unlike the character he played on TV (and for which he will most likely go to his grave being remembered for unless he stays away from roles like this one). Also, as a protagonist, he's almost entirely inactive during the bulk of the action, basically just sitting around and acting as a punching bag (literal and metaphorical) for nearly everyone else in the movie. When he occasionally does take some kind of action (such as a more or less entirely unmotivated peewee football game, and later the climactic bungled staged burglary), he flubs it egregiously, and usually because of his own ego. Thus it becomes really, really hard to sympathize with or even like the character, which is pretty important in light romantic comedy. As an audience member, I kept wondering why in the world does his fiancée say she loves him and only find happiness with him, when the closest he can come to being charming is a half-assed paraphrasing of dialogue from 'Jerry Maguire' (this before he proposes to her)? I mean, what a freakin' lame-o.
Second big mistake: why reveal that White's character is a cop the minute he's introduced to her parents? Look, you've got a potentially really funny set-up, with Clifton Powell as a cop-hating former Black Panther and his future son-in-law as a policeman wanting to impress the old man favorably, so right out of the gate there's a terrific source of comic tension, where you could have White's character running around for the bulk of the film trying to conceal his job from Powell and getting into all kinds of trouble as a result (there are some hints as to how this might have developed in White's initial interactions with the character of June Bug, but that's quickly and inexplicably defused - good job, 'Coke'). Instead, first thing that comes out when he meets his fiancée's parents is that he's a cop - no warning from the fiancée that, "You know, uh, by the way, my dad's a former Black Panther and he hates policemen, so maybe you shouldn't mention that to him, okay?" And then the father's reaction to this is so implausibly and overtly negative that it goes way, way beyond any kind of risibility into outright unpleasantness, not to mention complete unbelievability.
Which is another of this film's greater weaknesses: all kinds of baffling incidents of "What the..?!" implausibilities. Like the fact that White's character, a uniformed beat cop, has his own desk at the police station and apparently is allowed to just kick back there whenever and yack with his fellow officers who also apparently have nothing better to do. Or that June Bug cannot recognize fellow gangstas as friendly until they're within five yards of him. Or that the guys with whom Powell and White are playing dominoes in the park would make such outrageously crude and grotesquely sexist remarks about Powell's wife and daughter when it's obvious to even the biggest idiot that they're just that: his wife and daughter. I could go on and on, but my point is that if your story is set in something at least resembling the real world and, more importantly, we're expected to have some kind of emotional involvement with the characters, then there has to be some level of believability as well as psychological consistency to said characters. That just ain't the case here.
As for Urkel's boner, let's just say that there's nothing quite so disturbing as a Jaleel White sex-dream followed by a Clifton Powell wake-up call.
