Yeah, I'd think long and hard about this relationship. He couldn't care less about your feelings and needs obviously. That's no way to live.

They've even taken my toothbrush and use it to brush the cats teeth

This would be my ticket out the door.

Why do you allow your boyfriend to treat you so poorly? Is there something so great about him that makes it worth putting up with that nonsense? The behavior you describe is not that of a man who loves or respects you.

Move out, or if it's your place, kick your BF out.

Easy solution:

The kids can't take your stuff if they don't have access to it, and they can't have access to it if you don't date their dad anymore.

This isn't something you fix. This is something you say, "wow, this guy sucks at being a BF and I really shouldn't date him". You CANNOT have a successful relationship with someone who doesn't make their kids respect your stuff.

When DH and I were dating, we went shopping with the boys for something mundane. I paid cash at self-checkout, and YSS (he was 7 or 8 at the time) took the change out of the machine and pocketed it. He just grinned at me, likely because he thought it was funny.

I played along and held my hand out for him to give it to me. He shook his head.

My DH snapped at him to give it back immediately and, in the store, told him how he is not to take things from people, including me. He sheepishly handed me my change back - a total of about 9 cents.

THAT is how someone who loves you should treat you. They should want you to be respected and enforce respect by their children. It wasn't about the money or how little it was; it was about understanding respecting that it was MINE and not his.

The boys know which couch they can sit on when we're all together. They know that they can't just lay across everything if I want to sit down. We have several throw blankets, but one is mine that they don't use. They don't enter our bedroom (and I don't enter theirs unless it's an emergency). If I say don't eat or drink something, they don't. They ask before they use anything that isn't theirs.

They KNOW better because their father TAUGHT them better. Your BF hasn't taught his kids anything, and he isn't going to if his reaction is to get mad at you when you as for a BASIC REQUIREMENT of a relationship. He's not ready to be in a relationship.

My ss  who was 18 at the time use to take my stuff too. He would take my music cds and end up not putting them away so they would get wrecked, use my towels, use my toiletries. Once he stole my expensive body lotion and it was found in his room bedside next to used tissues, just yewww! 

The difference is my DH did not get mad if I said anything. Since most of the time DH never had the balls to say anything I would handle it myself and DH stood out of my way on that and would side with me.  If he did get mad and handle is like your SO we would not be married today. 

I suggest when and if you decide you have had enough, use all of their toothbrushes to clean the toilet on your way out. 

My SD not game enough to touch my stuff. Odd occasion it does happen I kick up a major stink stand my ground. And if my step kids took my tooth brush too clean the cat teeth I take theres when no one was looking and clean the toilet bowel with it and put it back afterwards like nothing happened. Evil I know. Sometimes they just drive you to do these things.  

people and their boundaries meaning their belongings and that you need to ask.

heck  i told hubby his son had no shame demanding to his dad how he was taking my fancy touchscreen laptop because he broke his for the 5th time in a year. Did i feel bad about it? Of course not!!

i reminded my husband that since ss says me and our kids together are strangers then strangers don’t share things...

"They've even taken my toothbrush and use it to brush the cats teeth. "  all time best post EVER!!    I laughed about this all morning, picturing your cat all moving his head to the side and pushing skid away with his paws..meowing :   "GET ME OUT OF HERE!!!!!!!"      that cats going to hold a grudge.

1. stepparent rule is hide your toothbrush. 

I'd use his damned toothbrush to clean the toilet.

I threw a fit the other day because my SO let his SD drink the rest of my Starbucks without asking me. I can't imagine him letting her use my toothbrush or my clothes.

But then again, my SD(9) never asks to use my stuff out of just normal respect for other people's stuff. She won't even use her dads stuff without asking. Its ridiculous that your SDs have never been taught boundaries but then again, they still sleep with your SO (disgusting imo), so...yeah.

The one time I DID catch her secretly using my stuff, I rightfully scolded her and she never did it again.

I don't think telling you to break up with him or whatever is probably very helpful for you right now so my suggest is hiding things. I hide MY food, drinks, pens, etc. so their isnt even a chance for SD to get my stuff. 

He sees no problem with it.

And that, my dear, is what YOU should have a problem with.

There are better men out there.

to scoop the litter box.

with Nair.  Whoops!

Wowwwww.  Dealbreaker.