IME ( I don't have children, but have worked with teens and their parents for 13 years), it's OK to give input, and of course, in a serious situation (abuse, drugs, etc.), intervene, but the minute you tell them they can't date or see that person, they will dig their heels in and get even more serious.
I had a frantic mom come into my office once. She had found out her daughter, who was a great kid, had snuck her BF into the house. She was soooo upset about it, and couldn't even talk to her or look at her, she said. And then she told me that her daughter really wanted to introduce her BF to them, and she just couldn't bring herself to do it. I finally got her to realize that most parents were KILL for their teenager to introduce their BF/GF to them, and that was a really good sign. She finally calmed down enough to get it.