I adored my x-FIL. I didn't even really know him well until after I broke up with his son.
My x-FIL was a huge support for myself and my two children when I put myself through nursing school.
He kept us in old cars. Would take us to lunch, give us all hugs good-bye, and I would find 100.00 bill in my coat
pocket later.
He never bad-mouthed or under-mined my new relationships or marriage.
He was just an amazing grand-dad to my kids and friend to me.

If your fiance's x-inlaws are good to your fiance and supportive of his life with you...I don't see a problem.

Are they nice people? Are they polite and kind to you?

My DH is fond of his exMIL (and late exFIL). She is very sweet and I have no idea how she gave birth to such a cold fish of a daughter. She even told him off (very nicely!) in her Christmas card for not sending an annual up-date. And she always addresses the cards to both of us and when she sees DH she asks after me. A very nice lady who I could be friends with.

His ex in-laws ... He still sends Christmas cards and gets one in return.

BUT ... we do not hang out with them. When we drive through exMILs small town we do not drop by to chat. Our holiday appointments do not include his ex in-laws. When his sons were small and he would drop them off on Christmas Day he would stop to chat with them at his ex's place. But not for a meal. Not a planned visit.

I think you need to discuss this habit with him. How if he wants to spend 15-30 mins visiting with them that this is OK however hours spent with them and not you makes it clear he may not be ready for a wife when he still has ex in-laws he respects more. See, it isn't you personally. He doesn't love them more than you. He may love them more than anyone. And if this is the case he cannot remarry because that would just be unfair to any other woman. So he has to be honest with himself. And also you need to be careful not to be controlling and making him do it all your way.

My sister has remained close with her ex-in laws. They are good people and have helped my sister out a lot over the years.

I get where it could feel weird but as long as they are not trying to get your FDH and the ex back together, I see no harm.

Do they accept you or have you met them?

My first husband died. I always remained good friends with his mom and I still invite her and her husband to a lot of our family functions. My DH does not mind at all.

How do his ex in-law’s treat you?
I am still friends with my ex-MIL and see her several times a year, getting together for lunch, etc. She has hung out at my house with DD while DH and I aren't there. That being said, if she treated my husband badly I don’t think I would have that kind of relationship with her. As it is she adores my husband, and asks about him whenever I speak to her, and he thinks she’s great too. But, that’s just what kind of person she is. My ex-FIL and his wife, I haven't spoken to since I left ex-H, which is fine with me.
So, I guess my thoughts are, if they treat you like you’re a leper, or talk badly about you to your BF, then maybe he shouldn’t have so much contact with them. If they can move on and have a friendly relationship with him, and not dislike you simply because you’re not their daughter, then I don’t see any problem with him being in contact with them.

That's tough. Gosh.

And one of the not often discussed issues with divorce. I wonder how his ex feels about it...

I don't have a relationship with DH's grandparents (who raised him - parents passed away), I will make conversation when we visit, but they are enamored with the ex who does DH so dirty when it comes to SS8. She does not have a conversation with DH that doesn't eventually involve talking about the ex. She even does it when I'm around. I usually just leave the room. She has her over for dinner and everything, and then says it's to have a relationship with SS. Uhmm, he's 8 years old! Not 8 months old. His mother can drop him off and pick him up. But now, I'm 'whatever' about it.

When my ex and I "split" (dated, never married), I was still really close to his family. My kids were their first grandkids and I'd been like a big sister to his only sister (7 years younger than us). Eventually, he shared how much he hated knowing I could be at his parents house at any time and how awkward it was for him to think about bringing a date to an event because I might be there.

So I backed off. We aren't that close now but it seemed like the right thing to do.

Sometimes things get sad when we have to move on. If he's not willing to move on from them, that would be a deal breaker for me. I can see seeing them occasionally , but if he's fixated on them there's something else going on here. My mom's best friend's ex always spent Xmas eve with her mom and everyone until he died, but it was just once a year.

WHY did he and their daughter break up/divorce?

If it was a mutual and amicable thing, it probably isnt a big deal as long as the ex family treats you with respect. Plus if they are grandparents, then they do need to communicate with grandkids and by extension, your fiance.

I stay in very basic contact with my ex inlaws and family because my ex lives with them. So I see my former inlaws on an every other weekend basis to drop off my kids and my ex-MIL will still say she loves me. My own fiance is very well respected by them and they are very kind and warm to him. They appreciate how he is towards their grandkids and my ex is also very courteous and friendly to my fiance even saying once to me that as far as he is concerned our kids have two dads. I was rather floored. Makes things much easier for me and my kids to have that acceptance. Not necessary but nice nonetheless.

Perhaps you are frightened he's not over the ex wife/GF which is normal. But what concerns me is why your fiance is so adamant about this in the face of your anxiety.

WHY did he and their daughter break up/divorce?

If it was a mutual and amicable thing, it probably isnt a big deal as long as the ex family treats you with respect. Plus if they are grandparents, then they do need to communicate with grandkids and by extension, your fiance.

I stay in very basic contact with my ex inlaws and family because my ex lives with them. So I see my former inlaws on an every other weekend basis to drop off my kids and my ex-MIL will still say she loves me. My own fiance is very well respected by them and they are very kind and warm to him. They appreciate how he is towards their grandkids and my ex is also very courteous and friendly to my fiance even saying once to me that as far as he is concerned our kids have two dads. I was rather floored. Makes things much easier for me and my kids to have that acceptance. Not necessary but nice nonetheless.

Perhaps you are frightened he's not over the ex wife/GF which is normal. But what concerns me is why your fiance is so adamant about this in the face of your anxiety.

Can you think of them as just good friends of your husbands? Really isn't that all they are? And reality tells you they're never going completely away being blood relations to his kids after all.

Better for all concerned, including you, if you set your feelings aside. You see these people remind you that your husband loved (bedded?) another woman. Well you chose to be a second wife - or are about to and its a package.

Perhaps breaking up is the best choice. This will give you the opportunity to find a guy who is childless and you can build your own family instead of adopting one. Seriously maybe that's best.