You have to accept your parents for who they are. They are not always going to do everything you want them to or behave the way you want them to. The same goes for you with them!
Given that, since you are an adult, and you’re getting your education, it’s [i]very[/i] normal for you to have a boyfriend or to date. It would be unusual if at your age you weren’t dating while attending school. But, your parents don’t like that, and [b]sometimes it’s best to agree to disagree[/b]. Hopefully, the three of you (you and your parents) can continue to forge an adult relationship that is different than the one you have had as a minor. That means respecting each other’s differences and understanding that we all disappoint each other some of the time and the best way to combat that disappointment is to adjust our expectations of each other as we get to know each other.
Don’t get defensive with them. Try and understand why they feel the way they do. And tell them you love them, but you hope that they can accept who you are because you really want to have a rich future together and share your life with them. If they’re used to you putting up a fight with them on this subject, being loving with them will disarm them, and may change the dynamic for all of you.