Obviously you will have worked out that your existence need not indicate that your parents are very knowledgeable about sexual subjects. You will probably be aware of the level of their ability to speak on this matter and should take account of this in your approach. Many parents have difficulty with openly discussing sex. Try to understand that this will hopefully become acceptance given time, so be patient and try not to give way to anger if your news results in what you feel are hurtful responses. You must be prepared for the possibility that this shock may result in a reaction that you may feel to be rejection.
With this information we are trying to help you about how best to approach your parents with what maybe regarded by them as news they may well find shocking. How Do I Tell My Parents I'm Lesbian or Gay?