This situation isn’t simple. Yes, your parents aren’t being supportive. But your boyfriend has a part in it. It was certainly inappropriate and immature to be arguing with you in front of your family. The fact that it got physical with your brother worries me. Adults can disagree without getting physical. I can understand why your family is concerned.
You say you love this man. But you’ve been with him since your were only 17. It’s important to consider whether you are in love or in “habit”. Because you are used to each other doesn’t necessarily mean that you are meant to be together for life. He may make you happier than you’ve ever been but at your age you haven’t been an adult long enough to know how happy you could be.
Based on what you shared, I encourage you to take a big step back and to think about if he is a man you would choose now. Give yourself time to develop other relationships so you are choosing from a pool larger than one. Your parents are more likely to be supportive if they see that, after a reasonable break, you and he have grown and still want to be together.
I wish you well.