YES!!!!!!!!! it is VERY bad!
1. you're only 12; you're not considered a teenager, not that having sex when you're a teenager is great either. it's pretty stupid having sex at a young age.
2. you're going to really regret it.
3. you have to be emotionally and physically ready
4. you might think the guy loves you, but really he's going to dump you when you are finished the act.
5. doing it at a young age shows you have no respect for your body nor yourself
the list goes on and on....