Since you're 18, you're a legal adult now and you really don't owe your parents any explanation.
Why do you think college will be any different than high school? It isn't really much different.
Employers do look at dropouts very negatively. If you can't finish school, how can you possibly hold down a job.
If you're old enough to make the decision to drop out, you're old enough to listen to others tell you the implications of dropping out.
I'm not lecturing you, you're a big boy now and you should make your decisions as an adult, and now that you are an adult, you need to act like one, take control of your life, and move on and do what you said you're going to do.
Don't you think your parent's have a right to be upset about this? They know what the future looks like for drop-outs, and it's not pretty! Those with the actual diploma will get chosen first, for jobs and for college.
GED is NOT the same as a diploma. GED=dropout, especially for someone in your age bracket.
Just a few things to think about. Good luck to you, you're going to need it.