I  would make it a condition for getting married that you move to a "yours" (plural) home BEFORE the wedding. Don't move into his house as things stand because nothing will change. What you need is a 2-bedroom place with a bedroom for you and a guest room/office. Give the kids plenty of time to find alternative solutions and move out. This is the ONLY way you'll ever get them to leave. A second condition for getting married is that none of them gets to move into your new home or you will LEAVE.

It would be foolhardy in the extreme to move into your fiance's house expecting things to change.  I would not rush into marriage with this man, either.  He has shown himself to be useless at setting any boundaries for his adult sons, two of which should have already moved out into their own places.   Your fiance is behaving more like their obsequious servant than their father.  The thing about SS24 showering in your en suite was ludicrous!    I think Winterglow's idea of getting your own marital home is a good one, but personally I would not have marriage high up on my to do list at all.  I'd leave it another year and see how you feel.  

Do not move in unless you want no privacy and to be a servant to his kids. Date this guy if you want, but do not marry him until all the kids are out of the house and fully launched. 

There is absolutely NO WAY that I would move into that house. Nor would I subject my son to it.

I am in a similar situation and currently working towards establishing boundaries that i can live with. It hasn't been easy and we aren't there yet.

My conditions are no BM in the house (and not in the driveway/yard unless it's necessary) and a kid-free bedroom. It's been almost 2 years, and we still aren't there yet. It was hard, but i had to clearly spell out what i could and couldn't deal with, and i give no slack when a boundary is violated.

If i find one of the kids in the bedroom, i tell my SO calmly that they are there. It is then on him to enforce the rule. If SO is in there with them, i calmly say "i thought we made the decision that the bedroom would be kid-free." If i see a suspicious pork chop in the fridge, i say "did BM come over and bring food again in the middle of your custody week? I've told you I won't move into a house where she comes and goes as she pleases." 

I have decided i won't even think about moving in with him until these 2 simple things no longer need to be corrected regularly. These are my deal breakers. I will keep trying as long as i see effort from my SO or until i get tired of being the refrigerator and bedroom police ("And WHERE did this chicken come from?!")

Don't move in until he shows he can make and stick to the bare minimal changes needed for you to not be miserable living with him. He may say he will change after you officially live there. He may say things will be different if it's a new house. Don't believe him. He is a weak parent and you need proof he will keep his word. And BTW all those kids are old enough that pretty much any boundary you set for their behavior in a home you share will be appropriate, up to having them live elsewhere for all but the 17-year-old. 

Walk or keep living separately. 

ALL of those 'kids' are old enough to cook their own food. 24?? Shoulda been cooking for a good 10 years by now. Dang.

even my husband finds it 100% inappropriate for a skid to be in our bedroom or bathroom. 

The fact your partner says they are free to use master bathroom or chill out in master bedroom, he needs to remember you both are not family and it’s inappropriate to toss you to the side for ss which is what he is doing

Listen to your gut. It's your warning system, and it's telling you NOT to marry this man or move in with him. If you do, you'll be miserable and subjecting your son to a lot of upheaval.

This man isn't doing the work of preparing his life and home for you. He isn't standing up for you or insisting his sons treat you with respect. He isn't able to draw even the smallest of boundaries with his rude sons, and has shown he wants you to play servant to them. Their lifestyle is unstructured and chaotic, which doesn't seem a good fit for you. He's a weak, guilty parent and he wants you to accept poor treatment from his kids, just as he does. 

 Cr@ppy parents don't make good partners, so this man doesn't sound at all "amazing" to me. Have you brought up your concerns with him? What does he say, and has there been any improvement? Have you stood up for yourself, or are you just going with the flow?

There are huge red flags flapping all over your relationship, but as Rumplestiltskin points out, you will never have more leverage than now, so use it. In steplife you have to be able to stand up for yourself, speak up, draw and hold hard boundaries, and not accept poor treatment. Your fiance is showing you that he is a doormat and expects you to be one, too. Now that your eyes are open, start drawing some boundaries with this man. If he cant/won't hold his sons to basic standards of behavior, protect yourself and your son. DO NOT MARRY unless he demonstrates that he is able to parent effectively, or you will regret it.

This has so many red flags it could be a Chinese Communist Party parade.

Your SO wants a live in beck and call girl for he and his adult sons. One with benefits for him.

Do not ruin your own child's life by exposing him to the shallow and polluted gene pool that created these three failed men.  And for F sake, what would possibly be appealing about this idiot failed father?

And no, you shouldn't walk away.  You should RUN!

My husband asked me to do this and cook ss favourite dish which was focaccia bread. I had a daughter already and said to hubby one day “eff this, your kid is 18, abusive, shuns us and pretends we don’t exist, doesn’t acknowledge us, treats me like a maid and tells hubbys family that is my job (they told him off immediately and to lose that smug face of his when he said that), then when i cook dinner with a newborn, hubby tells me to knock on his sons bedroom door and ask him to eat dinner and ss won’t even open his door he keeps mumbling stuff so i firmly told hubby “you have a rude disrespectful pos for a son and on what planet is it acceptable that he treat me like this and you enable and encourage it?? Then you have the nerve to demand i cook dinner for him and knock on his door and be speaking to him for several mins trying to get him to eat dinner?? I am not his little bitch maid!! I cook what i want and he deserves no special treatment from me and you have no shame to demand it!! I cook what i want and i put it away, its upto ss if he gets it from the fridge to reheat but he has no shame to be eating any of my cooking with the way he behaves”

now when hubby gets home from work he forces ss to get out of his room to eat dinner and ss will eat on his own. There is no “thanks for dinner” he eats and goes to his room... manners do not exist with my skids but they are not a reflection of my parenting but their bi parents. How is it my 4.5 year old is constantly asking “may i have this”, “please” or even “thankyou” and adult teen skids are incapable of it??

If you want to continue a relationship with this man. That's up to you. But, I would not move any further forward in the relationship as things stand right now. As a matter of fact, I would consider taking a few steps back and insisting if he wants to see you, it is at your home where you can be comfortable.  You have made him aware of your boundaries so it it up to him to make changes if he wants to be with you.

I am always fighting with my bf about bedroom things. For me bathroom is fine. But when it comes to bedroom - I am not letting for ANYBODY  go in and especially to sleep in my bed except me and my bf..and we are having fights all the time about this.....

My skids are nearing adulthood, SS15 and SS16, and there is no way we will be letting them come and go as they please between our house and BM's house when they are adults. 

They don't have house keys now and won't be getting one unless they choose to live with us full-time after finishing high school (custody is currently 50/50). We expect the kids to choose a house to live in upon completion of high school and any visits to the other house (whether it's for dinner, a cuppa or staying overnight) need to be pre-arranged. 

We are expecting staunch opposition to this plan from SS16 and also BM. SD16 does not like making decisions or boundaries. She will want to change households on a whim, based on where she gets the best deal on any given day and BM would support this. But adults don't behave that way and we won't be allowing her to behave that way.

My hope is that she chooses to live with BM when she finishes high school end of next year as I honestly could not stand having her live with us full-time. SS15 will end up living with us full-time at some point and that's fine with me. But if SD decides to live with us (which I think is unlikely) then I'll make sure I don't make it easy for her ie I won't be doing her washing or any chores for her like changing her sheets etc and if she isn't studying full-time then she will need to work and pay board.