Why does a married man want to buy a house with his mother & her BF? Than tells you if you don't want to go it will end your relationship?? Something is off here and if he's giving you an ultimatum like this than I'm sorry to say but you don't have much of a marriage to begin with. He's dismissing your feelings and concerns on the matter. 

Who wants to purchase a house with their mother-in-law? That's a recipe for disaster and as a married couple what happens to your privacy. This is not a good arrangement. So what are you going to do?

I have no idea why a grown man would want to buy a house with his mom. The things that come to mind . . . He's a momma's boy; he can't afford to buy on his own; mom can't afford to buy and he's trying to help her out; he wants help with his kids and his housework.

If my SO wanted to live with FMIL, that would get a hard no from me. If he said that would be the end of the relationship? Oh well, how good of a relationship could it be if he'd leave you over you not wanting to live with MIL?

He decided? That’s not how marriage works. He asks you and only does so if you both agree. I’d never agree that this FYI. Sounds awful. 

What the....?

He doesn't view you as an equal partner. He will probably never make you a priority. If my DH wanted to buy a house with his mom, that would be a deal breaker. 

If he dies, mom gets the house and you're gonna get booted off the island to fend for yourself. Do you earn an income? I'd have a really hard time paying a mortgage for my MIL to make my life a living hell. 

You're young. Get out while you can if you're not okay with living with a mama's boy who will always put mommy above you. Don't settle for this BS. 

He's threatening that it will "end your relationship" because you're young and likely less experienced than him with relationships. He thinks you'll stick around and be responsible for half the mortgage.

Just say no. No, you don't want to do this. No, you won't do this. If it ends your relationship, so be it. Run to the nearest attorney and file for legal separation so that he can't get a mortgage and leave you on the hook for half of it.

He thinks he has you cornered. He thinks he can tell you what to do. You can allow it or tell him no and see how he reacts. If living with your in-laws until they die seems like the worst existence imaginable, don't do it.

DO NOT LIVE WITH THE IN-LAWS! That's my advice. Did it for a year and a half. Would NEVER do it again. Ever.

This would be the worst move of all.  Your marriage will be done for in that happens. 

How old his he and how old is his mom? 

My MIL is 67 and I am sure that within 10 years she will be moving in with us or my SIL. We would have an in-law suite though. I adore my MIL and have no issues with this. 

Since you do though, and rightfully so!! Just say no. Nope. Nada. Not gonna happen. Etc. Just keep saying that over and over. If you DH goes through with it, don't move with him.

Oh god no!! I couldn't give a bigger no! And if me saying no was the end of the relationship, then so be it. What is his reasoning for wanting to do this? Is the MIL elderly? I know within 10 years, my mom will be living with us (she currently lives alone and is starting to have a hard time keeping up with her house) but I hope by that time we will be able to remodel and her have an in law suite (I think someone mentioned this above).

I moved in with my father (took early retirement which cost me a lot) to look after him when my mother died.  He was still able bodied, but just wasn't used to paying bills plus his SS was only about $700 a month. (house paid for).  I took over all expenses.

BUT - I was single.  This was my decision and didn't affect anyone but me.  I would have never expected a partner or husband to live there too.

I am in my 70s and having some serious mobility issues.  If something happened to my DH I would move to a retirement apartment community or even assisted living if I needed it.  But I live in part of the country where I could live in a great place for what my SS pays.

Have loud passionate sex any time the Skids or ILs are around.  That ought to get the point across.

No.. is a complete sentence and on this topic is a complete discussion IMHO.  Not no, but hell no.  

We have lived with my parents for a few months or more a few times over the years. Either when we were building a house or between international assignments while our home was rented.   I have no problem with it and generally neither does my bride though she does get to where she wants her own space much sooner than I do.  I refuse to even spend a single night at any of my IL's homes. My health is more important to me than being in one of their homes for more than a couple of hours at a time will allow.  Their homes are dirty, allergin infested hazzardous waste zones.  When I enter I grab a hard wooden dining chair and that is what I sit in.  Except for BIL1's house. I will sit on their leather furniture.  Their is the only home that is not mold infested though they have a house full of cats and cats will shut down my respiratory system almost as quickly as mold will.

So I just won't stay the night.

Stick to your guns.  No, means no.  You and your DH need your own home that will allow you to focus on each other and your marriage.  A Gramma shed in the back yard is about as close as I would ever want to be to living with my MIL.  I would consider a multigenerational home with my parents but it would have to be purpose built with a private wing for each couple.  A common kitchen, dinning room and family room would work but each private wing would need its own kitchenette, small family room and of course bed and bathroom.

We have looked at homes that fit that model with my parents and may do it some day.  Not likely, but possible.