Get over yourself. Your parents are having sex, whether you like it or not. And why shouldn't they. Can you honestly say that you can't imagine yourself not wanting to have sex at their age? When you think about how long you'll actually live, can you honestly imagine that you won't be having "S-E-X" during the last 20 years of your life? C'mon! There was a time when you would have been happy to have sex 20 times a day if you could manage. So get a grip and stop being so afraid to talk to your parents about grownup sex.
Are your parents still together? If not, are they dating? Did you know that older adults and senior citizens are showing massive spikes in the number of STD and HIV infections? What we often forget is that older individuals never sat through high school health class or had sex education. These are the remaining leaders of the free-sex movement. Condoms weren't used, and if they were it was to transport cocaine in someone's stomach from some South American country into ours.
The greatest revenge we are naturally granted in life is that we have the pleasure of raising our parents as they enter old age, just as they once raised us as we grew from youngsters. Be the mom (or the dad) in the situation. If your parents are dating or having sex, be the responsible one that gives them the 411 on safe sex!