Tell them it helps you deal with people calling you a loser when you smoke up..lol...just kidding.
Bud, I wouldnt react that harsh when my kids start tokin ( callin you a loser is low and nasty )but I would be concerned.They are just takin the tough approach to force you to stop.
The biggest concern I would have is "have I taught them to be responsible enough to use without overusing?" and that is my position. Lots of folks who are against pot are so because they have been taught to. If you want to smoke and keep things cool with your parents then it is your responsibility to show them that #1 its not the reefer madness junkie drug they think it is and #2 you ARE responsible enough to handle it.
So my advice is approach them when you are not stoned and they are not pissed off and ask them what exactly their concerns are and their issues with pot. Then make a point of showing them that their concerns are manageable. Educate them on pot intelligently ( use any number of online resources), dont let your responsibilities slip, dont overindulge when you need a clear head, keep your grades up etc.
Basically re-teach them that pot can be less harmful and more manageable than alcohol and its not going to ruin your life. Fighting and arguing will NOT prove this point to them and will fuel their views that pot is ruining their baby boy.
good luck and peace.