If you and your parents are having a difference of opinion about the person you're dating or engaged to, you are probably WRONG and your parents are right. Not always, but that's probably the case 95 percent of the time. Why? Because unless your parents are evil, psychos, bigots, nutcases or drunks, they only want to protect you from your silly self. Your parents have been around longer, they have more relationship experience, and unlike you, they aren't horny for this person so they are more objective. Unlike your parents, you may be too young, blind, desperate, or frightened to be thinking rationally.
Just because you're enthralled with someone doesn't mean it's a good project for the rest of your life. I can't tell you how many times callers on my show have told me they saw the red flags when they were dating and people told them marrying their spouse was a bad idea, and yet, they did it anyway. Don't walk into the Valley of the Shadow of Death, have three kids, and then decide that everyone was right and you should have listened. Not all things in life are fixable; in fact, they can get a whole lot worse.
For example, one of the biggest warnings people ignore from their parents is about dating someone who is a different religion. A lot of people think religion is not an issue when they're head-over-heels in love, but it is an issue when the kids arrive.
I once gave a talk to a Jewish group at a synagogue. During the Q&A, a man stood up and said that he had married a Christian woman who had agreed to raise their kids Jewish. The problem was, they got divorced. Now he was upset that she was raising the kids to be Christian. On a side note, this is why a lot of kids grow up to be atheists. They figure that neither parent cared enough about their religious point of view to create a family around it, so why should they?
The bottom line is, you have to ask yourself if it's worth all the stress and drama for the rest of your life. If there are some personality issues, it's a good idea for everybody (boyfriend, girlfriend, and both sets of parents) to sit down and talk openly without getting defensive.
So, when your parents tell you that the guy or gal you are dating is a loser, psycho, drunk, cheater, or you can't count on them, you need to stop and say to yourself, "My parents are good people, and I respect them. There must be a good reason why they don't like him/her." And if you're dating someone who is rude or acts superior to your parents, dump them. These are the people who gave birth to you, nurtured you, raised you, and protected and provided for you.
Remember: The Romeo and Juliet story is very romantic, but people ended up dead.