Bad side effects as you call them are
1.You'll need it all the time.
2.You can get STD's
3.You can soon become a father.
Sex is something that comes for the body, mind, and soul. If you need to ask this question then it advise you don't even understand what you doing and getting your self into. Your way to young and so is she, your not even old enough to work, how would you raise a child ?(yes, condoms to work but not always) do your research then you'll know, google std and "side effects of sex ". As you get older you'll learn about sex, and you'll know when your really.
My advice to you is don't give in. Take time to grow into a young man, your still a child your self.