I'd only add that Master cried because he was overwhelmed during his first sexual experience, not really from jealousy. It was actually comedic since girls are normally the ones who do that in these stories but here it was flipped, with Sasha taking care of Master.
Also, Senpai is finished either way since Sasha doesn't like him like that. She already dumped him a while ago (which he complained about) and only did the threesome because Master was involved. As you said, she didn't even pay much attention to him.
He's a gag character at this point. How he's jealous and distraught because he is intimidated by this younger boy's "stamina and technique" was funny. He catches nothing but losses all the time, this chapter and the one where he got tied up and ignored by Master and Sasha after his supervillain monologue or when Master made fun of him by mentioning that Sasha must not remember him much because she never recorded him, etc.
I do agree, though. As Master gets more comfortable, the series is setting the stage for him to potentially be very "skilled." I'm sure he'll have a lot of ideas for his Ecchi manga career, no matter what happens.