I don't think anyone here is really faulting what you've actually done, which is have a conversation with your husband and work out steps to make sure it doesn't happen again.
What I think people are taking issue with is the level of anger and blame you're showing in your OP and subsequent posts towards your partner.
People are fallible. Not just men, but everybody. We're talking about a one time mistake here, not a pattern of negligence. He's not always leaving the gate open, he did it once. You seem to have an expectation that he never cock up, but that's never going to happen. He's human, not a robot.
You ask in my quoted post what other people do in similar situations, so I'll give you four examples.
DD fell off a changing mat on a chest of drawers while DP was changing her. DP had turned away for a second to pick up a nappy.
I one made a bad decision about which route to take in the car in icy conditions. DD was in the car. We very narrowly avoided a bad accident.
DP once lost DD for 20 minutes in a busy shop
DD once got injured in a water park due to my inattention. Not badly, but enough to put her off the water for about a year, and put a dampener on the holiday. She still has a small scar on her back 7 years later.
Now putting all of the above into 4 sentences makes us sound like awful parents, but we're not negligent, I promise! Those 4 things stand out precisely because they're so rare, and most parents will easily be able to point to that many similar things that haunt them at night by the time their kids reach adulthood.
But what we did in those situations is resolve the immediate crisis, make sure DD is Ok, and then make sure the parent who cocked up is OK, isn't too shaken up, doesn't feel too guilty. And then finally putting steps in place to stop it happening again.
There's no assigning blame. Yes there's blame to be assigned, in every case above one of us either made a bad decision or was distracted, but what's the point really?
Where's the need to feel that much anger over it? It's a one off mistake, that the other person has acknowledged and apologised for. What more is there to do than say "Oh well, only human" and swiftly move on - as you put it?