What should I do if my parents aren't fond of the guys I choose to be with?

I'm assuming that you are either under 18, or living at home here, as in other situations, although it may be awkward, it's really none of their business.
My first piece of advice would be to have a chat with them (depending on your parents you may choose to talk to both at once, or each individually) don't be aggressive or moan or do the "Its so UNFAIR" act on them. Just say calmly that you respect them, understand that they want the best for you and are willing to accept their rules whilst you live with them but in return you feel you are mature enough to at least deserve an explanation of what their specific problems with the guys you date is. You may find that its the idea of you dating at all that makes them uncomfortable: perhaps they worry that these guys will try and take advantage of you, or distract you from schoolwork. You may be able to come to some compromise about this- perhaps if you agree to curfews, or to spend a certain time of time working before going on a date, or not to have them in your room with the door closed. Try to meet them halfway.
It may also be a case that they don't really know the guys in question, and this might worry them (understandably- put yourself in their shoes. Would you want your daughter going out with a man/ boy you hardly knew?) Try offering to invite him around for dinner, or as a guest to a family gathering, let them get to know him and they may come around.
To illustrate this: when I was 15 I dated a boy with a motorbike and long hair. My Dad assumed he must be a bit of a drop out and a bit irresponsible. However, I invited him around at a time when my Dad was fixing his car. My boyfriend offered politely to help, and within a few hours my Dad realised he was actually a very nice, intelligent and respectful (if slightly scruffy!) young man. You may be suprised how well this works.

If there is some other reason, you may want to take time to consider it. They may have a point: if perhaps the problem is that the guys you're dating are too old for you, or have bad reputations, or are known drug users for example- perhaps they have a point it would do you well to consider. You may think parents don't understand- but perhaps their objective view would be a valuble one to consider in this situation.
On the other hand there may be some completely irrational reason (for example: he's of a different race or social class). Unfortunately there's little you can do whilst living under their roof. Some people are simply prejudiced, at least you will know where you stand if you ask. I would make it (politely) clear that you don't agree and leave it at that. I wouldn't advise dating someone against your parent's wishes, as it will put an extra unecessary strain on your relationship. Wait a few years and you'll be free to do whatever you choose.
Good luck. Hope this helps

If you're over 18, and no longer living with, or supported by your parents, then it's entirely up to you who you choose to date. If your parents strongly disapprove of the person whom you are dating, you might be wise to find out why they disapprove, and to give it some consideration. They are older and wiser, and have a lot more experience than they do; and they just might have some important insight that you lack. However, the final choice is yours, after all, and not theirs.

If you're under 18, then your parents do indeed have the authority to tell you who you may or may not date.

And even if you're over 18, if your parents are still supporting you, and you make choices with which they strongly enough disapprove; they might well choose to withdraw that support. Are you ready to support yourself without their help?

Certainly consider their advice, but if you feel that you and your parents can't agree on whom you should date, agree to disagree. Just be patient and hope that your boyfriend does prove to be right for you and that they can see that. If they don't, just remember that it is more important for you to be happy with whom you date than it is for your parents to approve.

Date guys that your parents are fond of because your parents usually right in who you should date because after all, they have more life experiences; especially, in the dating world!

Heed their advice V-E-R-Y carefully! Parents are SELDOM wrong on these things!