Sex is a wonderful act that can pleasure both parties in a relationship. It is a defining statement that both must be mature enough to handle. I believe it is something that should be waited on until that person it truly ready to take on the emotional impact of the action.
On that same note, this time will be different with different people. I do not view sex as a simple, in and out (pardon the pun, and for those who have seen Clockwork Orange, you will get even a greater kick out of that) procedure, but I also do not agree with the need to wait for marriage.
Use protection until you are serious about a child, and have some fun. That is all life is about. You only get one, use it wisely, have sex if you feel you can take on the responsibility, and if you are uncomfortable with it, then don't do it.
But also do not try and make your opinion divine and all important, as MiraclrPlz seems to like to do. Others also have the right to an opinion.