If they are in her room together, it should be understood that the door would remain fully open. This rule is not based on your mistrust of her moral character, or a denial of her sexuality or need for privacy with her boyfriend. It's based upon the understanding that when she is in her room with her door closed, that's her way of telling you that she needs privacy for certain reasons. Under those circumstances, you will agree to knock on her door and ask if you may come in. Be considerate of her need for privacy when she is alone, but not when she's with her boyfriend.
Discuss your thoughts about this with her. Ask her what her feelings are and listen respectfully to her opinions. Don't condemn her if she disagrees with you. You may respectfully disagree about many issues and she must always hear the reasons behind your limits and boundaries, not just that she is forbidden to do something because you "said so." Thanks for writing.