I would have a frank discussion with your husband and tell him you are NOT OK with this practice, and tell him you want them to give up their keys to your house.
It is your home as well as his, and you have a right to privacy of your home and possessions.

Change the locks. They have no right to be in your home when you are not there. I just don't get this. My DH's kids wanted keys, too. No way in hell. My dad's stepdaughter does the same thing-she does not leave a mess, but she lets herself in as she pleases. My dad is retired, and he never knows when she will come by. He has been complaining and her visits have been less frequent. I have a key to his house for emergencies only. I would never dream of letting myself in for any other reason. I don't drop by unannounced, either.

You need to set your DH straight-this home is yours and his; it does not belong to his kids.

Just wondering, is it different or would it be different for your own bio children...if they are/were adults? (Sorry, I don't know how old your bios are)

I wouldnt want anyone entering my home, using my stuff,making a mess then just leaving it either.

When I moved out from my Dad and SM's home at 18, I was told to turn in my key. My Step Sister (SM's daughter), when she moved out, was allowed to keep her key and still comes and goes as she pleases whenever.

It doesn't bother me really except for the fact my very elderly grandmother lives with them and visiting her is difficult bc I have to try and find time when my Dad and SM are home to do it which is hard. So I only get to visit her about every 4 to 5 months or so.

But I think it is important for you and DH to be on board with eachother and respectful of the other. Easier to say than to do, I know. If the kids are adults, they shouldnt be coming over and being freeloaders...especially when nobody is home!

Objectively looking at it, one would ask "why should SM's biokid get to have a key to the house and expect their DH's biokid to not have a key?". Objectively it is not fair.

Yet I'm sure SMs and SDs who do feel this way do have their reasons for it....whether they are fair reasons (the SK can't be trusted, is vindictive/malicious etc) or not fair reasons "but he/she is MYYYYY bio and SK is not and that's why").

It's tough. Luckily I have years before I have to cross that bridge. Likely I know how I would feel, but for the sake of keeping things fair when they are adults, and as long as SKs don't turn out to be a-holes or ones who can not be trusted, I would likely go along with either all kids being able to have a key for emergencies only, or none of them....ah hell, I'd probably secretly give my kid a key for emergency use only if that were the case

Maybe i'm too much of a hermit or too into personal privacy, but i wouldnt let anyone have a key...
none of us had keys to my parents' place after we'd moved out. actually i wouldnt have any valid reason for going over there if it wasnt to see them!
and i had enough sense (basic manners?) to even *shock* call in advance!!! i didnt ever feel like i had to ask permission, it was more of a "hey, u guys gonna be around for a bit? yes? ok cool i'll be by in a little while."
if i was watering the plants and taking care of the dogs while they were out of town, they'd give me a key, which i'd give back when they returned. it was the same way for all of us kids- both bio AND step.

None of us kids had keys to my parents house when we moved out and my parents didn't even lock their doors back then either and we only went over if they were home and invited us or it was a day anyone could stop in like on Sundays. We always called ahead even when mom said it wasn't needed. It was just how things were back then. Now a days kids are so much more less respectful of adults in general. We do not allow ss19 over here at all when we are not home, we lock the doors and bs15 and ss16 have keys in their backpacks. Even when dh is here ss19 knows to only step inside and sit on the couch to visit. He has stolen so many things from us that he is not allowed upstairs at all. He is on major secret probation and I am grateful that DH enforces this all the time. Your DH better start cleaning up or one day he's going to get home to changed locks and no way inside

Change the locks. Seriously, why are you tolerating this? I would have made a hell of a stink. I had this problem when I moved in with DH , with the BM and SK's. Had the garage door re-programmed and that stopped some of that. Also, made DH put alarms on the door since they decided to just walk in when they felt like it. CHanged the locks and SD did not get a key. SS did since he lives here full time. Told him if he gave the code or a key to anyone, he would have to wait outside until we came home. He has never had a problem with that.

I freaked out on DH when SS walked in one day while I was home recovering from surgery and he knew I wasn't kidding. He was on a ladder the next day, figuring out how to do it. I would have loved to have seen BM's face when her garage button no longer worked....

I have full access to my mothers house any time of day. Sometimes I drop by to pick things up that she's left for me, or to leave things for her. But really I don't have any reason to hang around if she's not home. It's not my house anymore, I don't feel comfortable lounging about without them being home, it's just weird.