I would like to add that a good story is a good story with or without lewd stuff in it. If your story DEPENDS on erotic material, it is not a good story but simply an excuse to show people pictures, animations or videos.
There is nothing wrong with an Only-Sex-Game. Not all games NEED a story. But if you want to make a story - why not make it a good one, right? So write the story first and add the sex later, wherever it makes sense and wherever the player might need an additional motivation.
About the other question: I wouldn't recommend such a complicated setup for a beginner. Four protagonists are a hassle to write for: You have to give them each their own personality and keep track of them, make them react to everything differently ect.
If I were you, I would either try a different story entirely or write it in a way that there is ONE protagonist and three side-characters / companions. Then cut out all content that is not important for the protagonist, focus on his story and use the other three to flesh out the world. Use them to explain settings, to react to what is happening ect.
Do not burden yourself with too much, especially in your first project. Follow a simple story. One arc, one character, one goal. Not many side stories. Make it short, but sweet. Add other things later, after your main frame is working out. Believe me: Your motivation will fall flat once you start working on a story, and if you do not have an end in sight, you will give up. Therefor: Short, but with quality.
For starters, ask yourself the following points and build everything else around it:
1. Who is your hero? What makes him special, what makes him stand out. Most porn games think that giving a guy a big dick is enough to make him the perfect protagonist - think again. Also, keep in mind: We are not defined by our strength, but by our weaknesses. Think about what your hero CANNOT do, think about where he is lacking. That makes him relatable, and it makes his victories and conquests even sweeter afterwards.
Example: Fucking the cheerleader is nothing special if you are playing the big-dick-quarterback. But fucking the cheerleader after you started out as a nerdy nobody, this is better. Still boring (the story got told a thousand times already), but at least better.
2. What is the main goal of the game? What is the END. Write the end! Do it. Write the prologue, then write the end. Write THE FUCKING END. Because this way, you take away the opportunity to move the goal post later on. You write the end, then you write the rest. It gives you a clear vision where everything is going towards.
Example: The goal could be to kill the villain. The goal could be to fuck the villain. The goal could be to rescue the princess. Ect. - don't be too smart for your own good. Keep the goal simple.
3. What is your hero's goal? This is important, because in bad stories, the protagonist often just wants what the story needs. But your hero should start out with his own motivations. Maybe he wants to fuck the villain, but he cannot do that because the villain needs to die a virgin?
Maybe he hates to be a hero? Maybe he just wants to go home to his high school sweetheart, but instead he is forced by destiny to be the greatest hero ever known! Flesh out your base material - if your DND-Campaign doesn't give you enough, add stuff.
Example: Your hero wants become the cities champion. In the story, he will go much further, becoming the champion of humanity, but he would rather go back to his smaller world because he gets easily overwhelmed. This makes him relatable, but also is a good reason for comic relief - only his horniness makes him go forward, otherwise he would just leave at any given opportunity.
Those three steps could be your way to bring order into the chaos. Pacing is something completely different, as described in the other post, but first you need to go through your work and cut away everything that is not necessary and wouldn't interest your audience. It's a cruel process, because you need to cut your darlings, you need to throw out what you love - still, it is necessary. Otherwise, you end up with a huge to-do-list and an unfinished game in the end.