I'm not at all the jealous type, but this is totally inappropriate. They are going to share an apartment for the weekend? It might be semi-appropriate if they each got their own separate hotel room, but even then, it's totally unnecessary.  BM isn't doing it "for the kid", she's doing it "for herself" and if your BF can't see that, he's a fool.

I'm all for civil co-parenting, but having to spend the weekend with BM would not fly with me. Ask him how he'd feel about you spending a weekend with an ex in an apartment, and would he want to join in? Just because a kid exists doesn't make it any better.

If your partner agrees and goes, then I would have to ask why did they break up?  BM is lonely and looking for love.  She got him once, she will keep trying until he shuts her down for good. He needs to set her straight now that it's over or you will be questioning all of this , all the time.  Did you want to be part of a throuple?  Deep down you know this is wrong, we don't have to tell you that.  Co-parenting can and is done all the time without sending the kid fake messages of family harmony.  Playing at family when there are other partners involved is not healthy for anyone, especially the the kids.  
 

Can he be loyal to you and keep his BM in her proper place?  If he can't keep that straight in his head, you have some revaluation of the relationship to do.  You are way too young to waste time in this situation, I don't care how magical he makes you feel.  There are other men out there that don't have this kind of baggage. 

This would not fly with me.  Early on in our relationship, DH would do things with BM "for the kids." I guess that was fine when he wasn't in a relationship,  but I saw no room for BM in our relationship. I told DH that if he wanted to go trick or treating with BM and kids, or go to joint parties, he was welcome to, but I would be walking away because it wasn't acceptable to me.  A trip away with BM would have made my head spin and would for sure have been the end of our relationship.  Boundaries are key, and if something makes you uncomfortable, he needs to consider your feelings.  What they are proposing isn't appropriate, IMO, and it certainly isn't necessary. 

Joint anythings are inappropriate. Tell him to think of his daughter and how confused the happy family guff will make her. How is it helping her to let her think her parents are together? How is it going to make her feel towards you? She's going to hate you because she sees you as the reason her parents aren't together anymore.

If he goes despite knowing how you feel, I'd be gone when he gets home.

No joint weekend away.  No joint party's .  Your DH must pick his relationship,  either with you or BM. Not a threesome 

This is inappropriate.  Sounds like BM is lonely and looking to get back with him.  Or at least hook up.  My SS'S BM tried this once.  She and her husband called it quits and she showed up at our door while I was gone.  My in-laws were living on the same property as us, so SS was playing with one of his cousins at MIL's, giving BM opportunity to try to hook up with my husband.  He was on the phone with MIL, who told him that BM was coming to our door.  This was after BM had asked SIL if dh and I were home.  Upon being told that he was but I wasn't, she told SIL that she wanted to get some of that, meaning my husband.  SIL told her something along the lines of if she did then dh and I would split up and I'd beat the hell out of BM, which I probably would have.   At the time, dh didn't know about that conversation but it didn't matter.  She came a-knockin, he refused to even answer the door, and she gave up and left.  He was about your boyfriend's age.  Just a cautionary tale, especially if she hasn't asked to do joint anything in the past. 

Even BM over here never even thought about doing joint anything with dh.  She would throw birthday parties for SS but we did not go.  I think MIL did a couple of times but we never did.  What's next?  Holidays?  Summer vacations?  No, start out however you intend to continue. If he can't respect you enough to not do this, you don't need him.  P!enty of guys out there don't have baby mamas at his age.  What about if she finds another man?  I would think he wouldn't like it either.  If he wants to do joint parties and stuff with her, he can just go back to her.  Him saying you don't care about his kid is just him trying to manipulate you into shutting up and going along with this nonsense.  Call him out on it.  

Haha! Oh, boy. Yeah, this would NOT be happening.

Wanna spend a few days in an apartment with your baby momma? Better take out a lease and don't bother coming home.

The kid has absolutely no memory of them ever being together, and in fact, will not even remember this birthday, so they can say it's for the kid all they want. IT IS NOT.

Please find someone more mature.

His X has zero place in a vacation property that you and your BF will be staying in together. BM can get her own room.   Your BF needs a smack on the back of the head to give him clarity that his failed past relationship has no place in his relationship with y ou beyond his COd visitation with his daughter. For damned sure his X has no place in your relationship.

This has nothing to do with his daughter and everything to do with his ball-less idiocy regarding  his X.

smh