You SHOULD have a say, and you should assert your right to have a say.  He should be asking if you are OK with them coming over for extra time, and if you say no, then he should respect that. And if you do agree to extra time, it should be clear that you will not be the person caring for them when they do come over.

Honestly, I would not have ever told DH his son couldn't come over, BUT - I did no parenting or care for him. And if I said, "that's fine, but I'm going to hole up in the bedroom and watch movies because I'm tired," DH would have said that was just fine and asked his son to be quiet and leave me alone.

You have to assert your needs as important too.

If you want to go out, you can go out, but you should also be allowed to have some down time in your own home.

My DH, never once, just unilaterally made the decision that his daughter was coming over. Yes, she was his daughter, but he chose to be in a relationship with me, so I mattered.

If people want to just do what they want, without taking anybody else into consideration, they should stay single. The, "but they're my kids," does NOT trump everything.

He knew when he got divorced that he wouldn't be seeing his kids everyday.

Personally, I am ok with DH taking SD(s) whenever he wants to, he's their dad. My only issues with this would be if it interrupted plans we had already made (unless it was an emergency) together.

As a SM, without kids, I get what you are experiencing.  But remember they are his kids. And remember they are not yours. There's plenty of times I have taken myself out to dinner with a book, met up with friends, holed up in the bedroom or on the back deck with a book, ran a LOT of errands that took ALL DAY, went on weekend get aways with my friends, etc when he has the SDs.  Because they are not my responsibility and I'm not their parent.

I would suggest you have a discussion with your DH, without judgement, and let him know that you will still be doing your thing if he has his kids.  If it's something you want to be doing with them, great, do it!  But otherwise, you have every right to opt out.  I do it to this day, and to this day DH doesn't like it, but hey, not my kids.

You'll have to learn to communicate effectively to make it work.

Does he expect you to entertain his kids?  I might have suggested that.. "hey, honey, I'm tired.. why don't you take the kids out to eat tonight.. and I am going to turn in early.. I can fix myself a sandwich".

I don't think I would have ever told my DH he couldn't have the kids.. but he wouldn't have taken them if we had some other "emergency/plans/issue" that would make it a problem.  If I was tired when they were over.. his job to keep them quiet and out of my hair...

My DH has his kids half the time, and I help out with them a lot, have a say in what goes on with them in our house, am able to discipline them, etc.  Given the choice, DH would take the kids anytime BM asks him to.  Early in our relationship he did just that and I went along.  (DH and BM are amicable and it's not unusual for them to switch up days or weekends to accommodate their various needs).  Once after we'd had them about 4 weekends in  row, BM asked him to take them again for the weekend and I told him I didn't want them to come because we hadn't had a weekend to ourselves in a month and BM was taking advantage.  He told her no and now he usually asks me before he agrees to take them on BM's time.  If he had not responded that way, I wouldn't have married him.  I have needs, too, and I do not think they should always come second to his kids.

My DH was just like your fiance when we first got together.  I remember all those feelings very well.  YOU ARE NOT ALONE!  I recommend having some discussions with him about what you are feeling and how you can work together and communicate together. 

And being tired of all the kids stuff - I totally get it!  And now my DH does too.  Sometimes I tell him when I just can't take anymore, 'why don't you and the kids go do XYZ and I'll stay at home.'  The kids are happy doing XYZ, my DH is happy because he gets to hang with his kids (and yes my DH ALWAYS wanted me to do whatever they were doing - 'so we can be a family' as he said, but you know what - not my kids!), and I am happy because I get to recharge.  

Remember - you are not alone!  And glad you said that you needed a little alone time!

Dump him and find a man without baggage. You’re not married yet, and there are red flags all over the place. Life’s too short for stepkid crap. Seriously. If I could have a do-over, I wouldn’t do it. At the very least get your own place to retreat to when he has visitation. 

You are not married yet.  It’s only going to get worst.  He does not respect you NOW is the best time of your relationship.  Only bad think can happen from this point.  BM could died, run off with new BF and you can get the kids 24\7 365 days of the year.  

Do you ever go away for the weekend alone.?  Do you ever do anything alone with out SK.  Plays coincerts camping .   Do you really want this life.  You are be treated second.  He is doing what ever BM wants.  BM is controlling your life and home, 

I agree with many of the sentiments above - these are your SO's kids, so if he wants to have them extra time, that's great, as long as it is not disrupting plans you already have in place, as long as he is not expecting you to take responsibility for his kids while they are with him, and as long as he is not expecting you to financially subsidize his time or additional activities with his kids. 

Finding the balance as a stepfamily is a challenge. Your SO needs to allow you to have your space, just as you need to allow him to do what he wants when it comes to his children. 

There will always be a power struggle when a new person (i.e. the stepmother) arrives on the scene. If your SO is truly interested in having a successful relationship with you, he will not let BM or his children call the shots about anything that goes on in your home. The two of you should discuss what's okay and what's not. For example, maybe last minute changes to plans are not okay or maybe unexpected arrivals from the kids are not okay. But, you need to reach that agreement together and figure out what will work for both of you in your home. Having strong, healthy boundaries will be key to success as a stepfamily. If there are no boundaries, there will be problems. 

The sad reality for stepmoms is that, especially if you don't have children, there is typically a power imbalance in the family. Your SO will always have the trump card, because he has to cater to the needs of his children. The challenge is that nearly everything can be spun as "for the sake of the children", so unless you and your SO are on the same page, there will be a lot of conflict over what are reasonable deviations from the plan and what aren't. For example, my husband's family and BM's family seem to think that it's in the "best interest" of the children to see both families on Christmas. This means I never get to see my family on Christmas, unless I go alone. As someone who always alternated Christmases between my mom and dad's family, I do not feel as if I was harmed as a child by not seeing some of my family on Christmas. I think this decision is purely for the benefit of all the adults in the situation and honestly, the kids would survive if they saw DH's family the weekend before Christmas every other year and the same for BM's family. Also, if SSs were my children, they would alternate Christmases. Yet, this has become WWIII with both DH's family and BM.

The biggest challenge to step-life is managing all the external people whose expectations about how things "should be" are not aligned and/or cannot possibly be met in real life. Many BM's feel that they should still be able to treat their ex-husband's like husband's simply because they have children together and often don't want to accept that the new reality means that the ex-husband has his own life and doesn't have to be at BM's beck and call. Managing this is trickier with children, but ultimately, issues arise when the parents have no clear boundaries.