Never have a relationship with (or date, etc) someone who you wouldn't hang out with if you weren't sleeping with (or trying to sleep with) them.
This removes all the problems of "opposites attract" vs. "common ground" types of rules for finding a suitable partner. Also, the above does not apply to people who are merely sex partners and not "relationship material" (though it might not be a bad thing to follow in that case, either).
If someone's personal beliefs or hobbies or interests would immediately disqualify them from being worth your friendship, to the point that you would not want them over with the rest of the gang for movie night, why the hell would you date them? The fact that they're cute and your parents like them is an antiquated notion; it's like choosing a husband based on the fact that he'd be a "good provider".
I'm thinking of someone specific when I type this, but I don't think they'll ever read this. Chances are that the people guilty of this are the least likely to recognize that they're doing it, although how you could not know is beyond me.