OP here. Well. I got a variety of responses, from locked doors being unsafe to my husband being controlling and maybe even a little creepy.
In my desire to have it confirmed that "I am right and he is wrong," I may have been a little unfair to my husband; "barging in" is a slight exaggeration.This all started to happen when my daughter built her gaming laptop when she was around 12; after that she spent a lot more time in her room with the door closed, laughing and carrying on, playing games with her friends and just "hanging out" virtually, I guess. To be honest, we weren't at all sure what she was doing, which is what bothered my husband, I think. He was afraid of her visiting inappropriate websites, playing video games that were too mature for her, etc. It wasn't really that he didn't "trust her," he just didn't trust her ability to always make wise decisions. I think a lot of what bothered him was that she was growing away from us; instead of hanging out downstairs with the family, she preferred to be upstairs in her room with the door closed. He would often knock on her door to "see what she was doing," and if she didn't say "come in" or open the door within maybe 5 or 6 seconds, he'd open the door. You could hear through the door that she was playing on her computer; she'd be talking and laughing with her friends, typing on her keyboard (she has a LOUD keyboard), so it would have been highly unlikely that she would be in a state of undress, but I still think you don't just open the door to your kid's room without their express permission. She then started locking her door, which of course made my husband even more concerned.
My daughter and I talked about it, and for her it was just that she wanted the feeling of privacy, the knowledge that no one was going to suddenly come in upon her. I completely understand that because I am the same way. It's kind of a "cocoon-ish" feeling, and it's really hard to describe to people who don't have this need. Like how extraverted people have a hard time understanding introverted people's need to get away alone and "recharge," the feeling of psychological safety some people get from knowing they are shut off from the world is hard to explain to people who like open doors. My husband just doesn't get it because he doesn't have this need.
He hates that she spends hours in her room with the door locked, but years have gone by now and he doesn't really bother to knock on her door anymore, he just texts her if he needs her. But he still grumbles about it occasionally. I know that it's something that he regrets allowing to develop into normalcy.
Our son just turned 14 and he is different from his sister in that doesn't seem to have that need for solitude, but he also built a gaming computer last year with his sister's help and has followed in her footsteps of now spending many hours in his room on this computer, and my husband absolutely hates it. He doesn't want to let what happened to our daughter (basically her spending almost all of her time away from us in her room) to happen to our son. But our son usually doesn't even close his door, much less lock it. It will sometimes be closed though, and this bothers my husband, and he WILL barge right in then, but again, we can always hear our son talking and playing games with his friends (virtually) so we can assume he's dressed and not up to anything private. My husband said the other day that he doesn't want to let this get out of control like it did with our daughter, that he doesn't want to see our son start locking his door. I disagreed and we had a slight argument, which is what motivated me to post this. I feel that if my husband continues to barge in on our son the few times he does close his door, then it's going to drive him to start not only closing it but locking it, and my husband feels that he there's no need for our son to close his door at all.