Tell your DH that one of your daughters has despoiled our bed and there will be no sex until boundaries are drawn and they know that this our space. Further explain that this is our private space and no one should be allowed to use it.

I've been in your position and it is amazing how teens that age will act like the queens of everything and dominate a space. They will also push to see how much they can get away with. I know you probably want to be non confrontational but it would be perfectly reasonable and pleasant to just go in and say lightly 'SD honey, could I have my bed back now please, you've got your own room if you want a nap'. In all likelihood she will get up without a fuss and wander off. If you really don't want to do that then it is also perfectly reasonable to ask DH to speak to them about your bedroom being off limits, it's your flat after all, but I would advise you to speak to SD direct and learn how to gently reinforce your boundaries, or you may find it gets harder to do so as time goes on.

I had to get my SO to kick SS16 out of our bedroom. He use to lie on the bed next to SO like it was just SO's bedroom. And I moved into SO's house. I kept at it and I did it, so now SS is not allowed in the bedroom at anytime without being invited. I also told of SS one time when he barged in. I asked him how would you like me to come into your room uninvited?
Get DH to set the boundaries, and if he doesn't, let him know that it's a huge libido killer to know that his teenage daughter has been sleeping in the bed you have been intimate in. Hopefully that will motivate him.

Read some other blogs about skids and the bedroom, do a search on here, there are some helpful comments.

You are absolutely not mean or selfish. Having someone else's children in the house is hard work, even if they are impeccably behaved and when they are teenagers it's even more intrusive because they want to do things their way. If you don't have children just the mess they make can be stressful. It's important to have some space to retreat to. To keep things amicable he ought to try to be understanding of that just as you are understanding of his needs as a Dad.

The 'I don't want to talk about it' attitude drives me up the wall, my husband does that too, it's the height of arrogance to shut your partner down when they have a problem or concern because you don't wish to face it. if something is an issue for one partner, then it's an issue for both whether he likes it or not. Why would he want you feeling stressed and uncomfortable when his children are there, all that does is set up negativity where there doesn't need to be any. And you certainly don't want him to feel under pressure, you just want a bit of privacy and peace and quiet in your bedroom.

You're lucky, you've got tenure and you're on home turf, a lot of stepparents fight out these battles in the bioparents home where they don't have the home advantage. In my very early days as Dads GF oldest SDs then 17, 13 and 12 once refused to put my favourite programme on the TV, which I had been waiting all day to watch, because they had taken the remote control hostage and were watching hours of Friends reruns. They had the nerve to suggest that I was being rude and could not just walk into the living room and change the programme. I was completely nonplussed, I had never come across such an attitude. When I went into the kitchen to discuss the situation with DH, who was making dinner, he just brushed it off as not a big deal, like it was perfectly acceptable for children to refuse a polite request from an adult. His solution? Buy another TV for his bedroom.

My BF/DH was paranoid about placing any limits on them in his home in case it made them feel unwelcome or not at home, without realising that in a real home there ARE reasonable boundaries, so not having any makes it LESS homely and normal. In a real home parents say 'oi, get off my bed' and no one gives it a second thought. If I had my time again I would be much clearer and calmer about stating and enforcing my boundaries with partner and stepchildren early on. Don't be guilted into thinking that you are controlling or unreasonable, talk things out from the perspective that you both have different parent vs non parent perspectives and expectations and need to accommodate each others priorities and feelings. His priority is having his children feel welcome in your flat, you try to support him in that, your priority is having some privacy and peace because that's important to you and helps you enjoy the time you do spend with his children, he needs to support you in that.