How long has the affair been happening? Because though it's complicated and you may not feel like it, you could be a victim. This is something you likely wouldn't recognize until you were much older, though, with older children.
Good for you for not considering an easy way out (abortion) because it really isn't in the long run.
Look, you are 19 now, an adult. You can run off for a year, give birth and then give baby up for adoption and not tell a soul. You could stick around and have the baby and refuse to disclose the father or say you don't know. Either way I recommend discontinuing your relationship with the father, the moral compass of any cheating husband is so out of whack it's horrible, and if you think you'll ever end up with him you either won't or you will and be terribly miserable and he'll cheat again with others.
If you want to tell them, you can say: Mom, Dad, I have something to tell you, and you need to sit down. Then they will need some time to digest the info and go through options in their heads of how to help you if they want to and to work through their surprise/anger/confusion or whatever they are feeling. It's up to you to tell them who the father is. But just because you don't doesn't change the truth.
IT'S GOING TO BE OKAY! This happens to people, you won't be the first or last. Right now, take care of you and your baby in you, and your best bet is to surround yourself with the support you need. I send you a hug. It's going to be okay.
AC has really good advice above. I stand behind that, too. Not telling the father and doing adoption might be the best for all involved.