Social Scripts for escaping an unwanted Sexytimes RPG
What I'm seeing in your question is that you don't have a D&D group, you have a sexytimes fantasy roleplaying group. Being in an RPG group that's not what you thought it was is a more basic problem* than getting your D&D players to change behaviour, because you don't have D&D players.
(It's not that D&D can't include sexuality, or even can't be totally about sexuality like this, but for your purposes “friends, let's play D&D!” didn't result in a D&D game as most people — and you — understood the proposed activity. You got something totally different.)
Break it down: you proposed playing D&D, but your friends aren't interested in exploring the wilderness, heroic adventures, fighting monsters and looking for treasure, debating queens and emperors — or any of the many things that a D&D game usually consists of. They strongly resist any inducements to adventure, and resist your attempts to reduce the sexytimes roleplaying.
So you have a group of friends interested in exploring their and each other's sexual fantasies, as a group.
That's the diagnosis of the situation. Now for the solution.
Since you don't have a D&D group, proceed accordingly
Do you want to have a weekly sexytimes roleplaying session with your friends? No rhetorical question implying inherent judgement there — it's the question you're faced with and it's a fair one. Some people might say “actually, yes.”
But I'm going to assume, based on your question, that your answer is a clear “no”.
Since you don't have a D&D group, you should stop trying to make your sexytimes fantasy roleplaying group play D&D. Accept that they already aren't playing D&D as you understand it, and proceed accordingly as if you don't have an established D&D group — you don't.
That's the conceptual shift you need to make. Now here's a social script that you can use (directly, or for inspiration) to move forward in the actual situation you're in:
Before your next game — another social gathering, a meeting to discuss the game, or over email, whatever works — talk to your friends about the weekly game.
Hey friends. I realise now that what we want from our roleplaying group is different. I want a traditional adventure game that's similar to fantasy novels and shows, and I'm not getting that. You want to have a game that's focused on sexytimes† — and that's fine. It's just not something I'm interested in doing.
So I'm going to step down as the DM. I want you to continue enjoying your roleplaying game, but it always has been yours, not mine. I may or may not participate — honestly, probably not — but in any case I won't be running it. That also means that I won't be trying to make the rules and accidentally cramping your style either (*grin*).
I don't know which of you might want the DMing role, but I'm happy to give advice to whoever ends up stepping into that job. If you're all thinking that DMing sounds no fun, well, just remember that the DM gets to have many sexytimes characters. Maybe it'll have benefits you don't expect.
So that's just what I wanted to say. I'm glad I introduced you all to something you enjoy so much.
Whether using that directly or something else, it's important to stick to talking about yourself and your wants, and what you are going to do — and when you do mention their activity preference, that it sounds generally happy for them. This helps you avoid going anywhere near accidentally being judgemental.
Not only is avoiding judgement philosophically important because hey, consenting adults can do what they want with their imaginations, but it's super-important for getting what you want, which is your friends' active acceptance of your feelings and choices for what you do next. Someone who feels judged isn't listening or in a position to be understanding of what you feel, need, and choose. You don't want them to fight about this, just accept what's already true — that you're moving on, and that's fine.
Now, that might be it. You've stated that you want different things, and (super-important) you've said what you are doing without implying that there's any negotiation, or that you need their permission. You're just an adult exercising your adult prerogatives to decide how you use your time, and you're not throwing them any shade in the process. All good!
So now what if they object? I can think of one that might have led to this situation:
“But roleplaying is about being able to do anything!”
Yes, sure, but “anything” doesn't always mean that the people in a group are interested in the same “anything”. No one game can contain everything, so you have to pick and choose what your roleplaying “anything” ends up actually being about.
But that's still a misconception I can imagine leading to your present situation, and if so, it might come up and it's good to have a similarly non-judgemental, self-possessed response lined up, just in case. So, another script to use or munge as needed:
Absolutely, roleplaying lets you do anything you want. What we want to choose off the menu of “anything” is just different, that's all. That's okay. Some people like to read or watch heroes being heroes, some people like to read or watch space adventures, and some people like to read or watch stuff with lots of sex in it. Our tastes in roleplaying don't have to match, just like they don't have to match in other entertainments.
So do you have any idea which of you might make a good DM? Or hey, maybe you don't need a DM. It's not actually a necessary job in some roleplaying groups. I can get you some resources‡ for that, if you like.
This responds to the misconception, again without judgement, and then (super-important) redirects the conversation to something that includes moving past this issue, so the conversation stays productively focused on what they want to do next, because you demonstrate that you're already decided about what you are going to do.
* There is an excellent article series on the basic problem of playing completely different games at the same table, how to solve the problem, and a group-discussion tool that's popular to help dodge the problem entirely with new groups (aptly called the Same Page Tool).
† “Sexytimes” is a playful term in my own social circle's idiolect. If it's too unusual for your social group, by all means use a more appropriate term! I chose “sexytimes” because, where I am and for us, it conveys lightheartedness without any suggestion of being mocking or judgemental. Any term that is neutral and light would fit my intentions in these scripts.
‡ i.e., ask us a question to help us help your new-DM friend or your new GMless-roleplaying friends. :)