Answer: Sex is like any other physical activity. As long as you are doing it correctly and for the right reasons, it will not hurt you. If you engage in a lot of sex in a short time, then you will experience irritation and soreness. The thing to be concerned about is, whether in five, ten, or twenty years you will look back fondly on the sexual experiences you have today. Make sure your needs are being met. If you are having sex frequently just to keep your partner happy, then it is probably bad. Quality should come before quantity.
From a health perspective, you should limit the number of sexual partners you have; the fewer the better. The more partners you AND your partners have, the greater the likelihood you will be exposed to a disease. You should always utilize safe sex practices, but even more diligently if you have numerous or multiple sexual partners. We are just now learning that some diseases are spread by kissing, and these diseases are linked to cancer. Sexual morality aside, multiple partners isn't beneficial to our health. We each need to decide how much risk is appropriate, and then life with the consequences of our actions. When we are young, we feel invincible, which results in us accepting greater than we likely will later in life.
Frequent use of your pelvic muscles should help strengthen them. You may want to consider doing Kegel exercises to improve your muscle tone and control. This will allow you to control the amount of friction between you and your partner's penis.
Sex is not bad for you, so enjoy yourself. Unfortunately, society labels women who enjoy sex, and who engage in sex frequently, in a negative manner. Society only wants women reproducing, not enjoying their body and life. Do not label yourself a whore or slut because you enjoy sex. Do not allow others to label you either.