In answer to your basic questions there is nothing that you can do to MAKE your parents like Steve, it has to come from their own attitudes towards him.
You Are Legally Old Enough To Have Sex
As a 16-year-old, you are legally allowed to have sex.
You seem pretty sure of your feelings for Steve, because you say you're in love with him, so I imagine that when you did sleep with him, you felt sure that you wanted to sleep with him, and felt comfortable doing so.
As long as he didn't push you into anything, there is absolutely nothing wrong with what you did, in the eyes of the law. In the eyes of your parents however, they believe that you shouldn't have slept with him because you weren't married, and they probably think you are too young. It will be very difficult to make your parents like anyone.
I lost my virginity at the same age as you, with someone that I loved very much, and still do love. I felt as if he was the right person for me, and I wouldn't have wanted to lose my virginity to anyone else.
I didn't tell my parents that I had slept with him, but I think they knew. They didn't say anything to me, because they knew that first of all, I was legal, and second of all, if they had told me not to sleep with him, it would have made me want to sleep with him more, just to rebel against them.
Try To Understand Their Point of View
Your parents cannot stop you from sleeping with someone at your age, but you need to compromise with them. Understand that they still see you as their little girl, and they probably always will. They see you as vulnerable to someone that they don't think is trustworthy, and could take advantage of you. Understand that they have every right to worry, they are your parents, and you are their daughter.
Talk To Your Parents
You need to try and have a serious conversation with your parents about what is going to happen in the future. You need to all promise that you won't shout at each other, otherwise talking will be pointless.
Tell them that legally, you are allowed to have sex, but you understand why they are worried about you, because you are still young.
Ask them to get to know your boyfriend a bit better, maybe have him round for dinner, so that they can talk to him, and see that he is as nice as you say he is.
Your parents are afraid of what they don't know, and they don't know whether you were forced into anything, or whether you used protection. Talk to them about contraceptives, and whether a visit to the doctor would put their minds at rest, so that you could go on the pill.
You Won't Change Their Views on Premarital Sex
It will be difficult to change your parent's view on premarital sex. There are a lot of benefits to waiting to get married, but personally I can think of a lot more disadvantages to waiting. If they refuse to compromise, then it will be difficult, but remember that it is your life, not there's.
The answer to your problem is all about communication and compromise. Let your parents know that you are having safe sex, and you're happy being with Steve.