You are smart - good for you. Two things wrong with him: 1) giving you an ultimatum rather than listening to how you feel about it and discussing it like an adult and 2) his attitude that clearly puts his future wife behind his almost grown and very unpleasant kids. Painful to break off an engagement but better than years of misery and eventually divorce. You are doing the right thing - hang in there!

Being cautious, and smart enough to not let yourself get manipulated will save you a lifetime of heartache. 

Its possible the nearly 18 year old could be there for a few more years yet (if they act like ‘best buddies’ you could probably add a few more years on that). 

It’s difficult for young people to move out, but equally some parents don’t give kids motivation to move out. 

I have lived with a manipulator, and these things can escalate to manipulation in other areas, and a lack of kindness and empathy can also accompany this (they can be very good at faking stuff in the beginning). 

Be careful is my main piece of advice. 

I would pause the marriage for now. 

You are not disagreeing about colors of bathroom towels. 

Don't put the cart before the horse. You will be unhappy I believe. 

Good for you! Also, this would be a non-issue if SS was planning to be a productive member of society at 17/18. Is he applying for jobs? Persuing higher education? Looking for roomates/places to rent? If the answer is no... you are dodging a MAJOR bullet. 

You have made the right decision so just stick to it - he tried to force you to have his almost adult son live with you on pain of not marrying you - fine - don't marry him!  Any man who would try and force a thing on his reluctant fiance is not worth marrying in my view. 

Let me get this straight. You have a right to have a home with none of his sons living with you.  Does he have that same right to have a home without your son living with him?

IMHO you would have been better to have informed your FDH that no kids would live in your marital home including yours, if you had any intent of having a marriage between equity partners. Or... you would recognize that kids are a part of a marriage regardless of the biology of that kid.  Yes, behavioral standards apply. But informing your partner that his children have no place in his marital home should be the end of that relationship before it even starts. All else remaining equal.

While I understand that his youngest will be 18 before too long, your ultimatum would have me take an even more severe stance than what your BF has taken. I would not have given you the ultimatum of my kid living with us or there would be no marriage. I would have told you that I would not continue in any relationship with you at all.  It is probably best all around to just move on.  If Skids are a deal breaker for you as far as a future marriage is concerne, then you know hte pool of candidates to focus on.  Men with no kids who don't mind that you have one.

Quality marriages are not built on ultimatums. Quality marriages are built on foundations of mutual respect. Quality blended marriages have the additional corner stone of the partners establishing and enforcing the standards of behavior and performance that children will adhere to within the blended family and home that are made by the partners.

Or, wait to get married until after the 18yo graduates from HS and launches.  Even if he goes to University his dad will in all likelihood want his son to spend school breaks at home which would have this kid living with you intermittently for potentially years.  You would be better served to discuss the behavioral issues that you have regarding his son and how to work with your FDH to address those behaviors within the context of the relationship.

Good luck.

Its strange as my SS lived with us for a solid month and he got in to a routine and adapted to our rules and how no meant no, we all lost weight, kid improved at school, improved attitude, improved general behaviour it was bliss but hard work. We even seemed to develop a better relationship. He couldnt wait to go back to his mums for the easy life though and when he did all the hard work we did was undone within a week and our relationship has just gone! Maybe ask for a trial before you both make a decision ur partner may not think its all crackedup to be or you may get lucky 

Mmm.. I would say thats the end. I am in similar situation. Just kid is small and kid is not moving in ( my bf expects that when kid will grow up he will want to move to us ( I am against that by the way)). All I see from topics here that men literally making crappy decisions in their lifes ( sorry...)you can not push away your wife or can not push away your kid. In order to make everyone happy kid suppose to live where he is living now and visit you guys as often as possible and spend time with dad. Thats it. In general that kid is already 17 years old......

Hope you followed thru and did not marry your fiance 

I've been in my marriage to my DW for 18 years now and SS is now 28.  He's with us now and I'm trying to get him to get a place of his own.  It's not good over here.     Hope you made the right decision.