I live in la la land - often leave the door unlocked and am fairly lax on security. I have had things stolen from outside of my front door - a lantern and stupidly, an inflatable canoe that I did not put away.
I think that if you are in the room, you do not need to lock the back door.
However, I think that you and your husband should probably speak about how you manage this interaction.
You don't think that you need to lock the door, so you don't and you feel infantilised by the constant reminders to do so - which you wilfully ignore.
He 'does not feel safe' if he thinks the door is unlocked and needs to have confirmation that the school drop off has gone OK.
If you were to lock the door, how would this make you feel? How do you think it would make your children feel? How will it make your husband feel? If you don't lock the door - how will it make your husband feel?
If you were to say - I think we will feel safe if the front door is locked and the gate is locked. If I leave the house or go upstairs, I will lock the back door - will that meet your needs? How do you think the conversation will go?
Can you have the 'drop off discussion'? Are you happy to confirm every day - that drop off went fine? Do you think he is sitting there waiting for your message, in a state of anxiety? Is it anxiety or control?
Are there other areas of your life / areas of his life - where you see this level of anxiety / need to be in control / fear? Do you know if there is a reason behind it? Has there been something that makes him feel unsafe? If you were to reassure him on some of these things, would that be enough? Would that make you feel locked in and controlled? Do you think talking to someone about his need to be in control / feel safe would help? What about your feelings of being locked in?
My take on this, from the outside... is that you feel that you can make an adult assessment of what is required to make you and the children safe and that this is not in line with what your husband thinks.... I think your approach is right - being in a constant state of thinking you are not safe, increases the levels of cortisol in the body and is harmful to our mental and physical health. Reducing the levels of stress being experienced by you and your husband is important and you are also modelling behaviour to your children. Again - I think your husband is likely to be exhibiting heightened levels of fear and not feeling safe, that are not warranted by the situation and that this is not healthy for anyone. I could be wrong and I see on there that lost of people feel that locking doors when you are in the room is the right thing to do... so maybe my levels of feeling safe are off.