if it was just a one off reaction then I would agree but he was always portrayed as envious of Susa’s sexual prowess, even when Miko isn’t around (the cosplayers/Iruka). He is also shown as genuinely shaken by the idea of Miko not desiring him like she does Susa.
Basically my working theory is Miko has been playing everyone’s envy off each other for her amusement and gratification.
She knew Susa loves her, Amane loves her boyfriend and that Saku has a frail ego. In Susa’s case she can string him along to keep sexual access open, knowing that fucking her is probably the biggest thing he has going in his life, and will probably always be unfortunately. That and sex with other loose cosplayers.
In Amane’s case she keeps her “dick” (Susa) warm and available when she need it by dating him, preventing Susa from forming relationships outside this weird little circle, as well as supplying an ego boost when she lends out her boyfriend to her frumpy friend knowing she would NEVER be a threat to her. Amane in turn gets access to the person she actually likes and Miko’s coattails as she desires to be popular like she was in her last college (as an easy lay for awkward nerds, re: Susa, honestly makes me think she never cared about Susa, rather she knew she could use sex to keep him around and fawning on her.)
As for Saku, he just boosts her profile and popularity. Association with the “godly artist” has done nothing but improve her career, him being attractive is just a bonus. Fucking Susa keeps him in line by playing off his ego making him feel threatened (you can be replaced) as well as stroking it (I fuck him, but I love you). supplying him with Susa’s girlfriend also keeps him frailty in check (see how much better than him you are, even sex with Susa doesn’t compare to just being with you), I don’t think Susa has shown once had any interest in Amane outside of swapping.
It’s really fucked up and again, a real downer. I don’t see how any of them can be happy in the long term, except for Miko who could probably yeet all three and live happily ever after.
Like, in three chapters it makes question if anyone ever really cared about anything but themselves and what they wanted.