Its good that your parents still have a strong sexual relationship, but thats not really the issue here.No, you are not sad at all.Its understandable that the thought of sex is constantly on your mind, you're a teenager! Having recently heard your parents going at it, of course it would stay fresh in your mind. For teenagers, especially at your age, thoughts that may seem disturbing and unnatural are normal and healthy. After exploring your own body it is natural to want to explore others. The key to surviving this awkward stage is to keep it in your head. I'm not saying that I think you would act upon them, but unless you have a very VERY good relationship with your parents, don't mention it to them.If you have any questions or anything, feel free to PM me.