You are not wrong....those adults should be in their own place

Yes, of course your wife should have talked to you about it first - what was she thinking?  This would be a deal breaker for me.  And yes, they do need to be in their own place. 

Been there, done that request. (At least, at the time my husband asked me.)

My situation was exactly the same, except it was to move in pregnant SD, equally unemployed boyfriend and their dog... I was done with parental insanity and enabling of adult children. Husband's insistence got him promoted to EX-husband. (His associated circus never got the chance to live under my roof or big top tent...)
 

Tell her you will be moving in your nephew, his wife, and their pregnant cat and see if she thinks that's OK.

My response would be that she has decided, since she could not do you the simple courtesy of even discussing the issue with you, to become responsible for the cost of her daughter, her boyfriend, and their eventual baby all by herself.  Best of luck with that!  Any joint accounts would be immediately severed and direct deposits terminated.  You are responsible for your percentage of the rent, utilities, ect.. only.  Actions have consequences.  Either she pays their way or has them pay it or they leave.  If she wanted you to contribute then you should have been consulted.  Wow,  She has some serious nerve!

That would end my marriage

Not wrong at all!!!   The pregnant 23 year old shouldn't even be living there let alone the boyfriend.  The more comfortable you make it for them the longer they will stay.  I am a firm believer in adult children do not live home or ever move back home.  

I think society is making it too much the norm for adult kids to live with their parents!!  

Look at that laundry commercial where the parents are folding everyones underwear saying 3 or 4 generations living under one roof ..... the norm ..... ummmmm NO !  Hahahaha

I divorced at 22 and I had a 3 year old I did not move back home!!!!!! 

Your wife is wrong, but I'm pretty sure you already know that.

Did your wife not consult you because she knew you wouldn't agree to it, or did she not consult you because she has no regard for you? Maybe you don't know the answer, but either way it's a problem that needs to be addressed.

You must get them out at any cost. They have zero reason to leave, get jobs or stop reproducing.

You had a bad enough problem with one freeloader about to reproduce in your basement to become two freeloaders. Now that the sperm donor lives there, you'll have another baby every nine months, and that will only make them all more dependent on you.