It’s been such a long time since I read this, but in one of the arcs it had the mc play a character who was obsessed with robots(?)/ mechanics, and one time the ML took advantage of him to touch(?) him in the while pretending that he was to help him fight(?).
"don't pick up boyfriends for the trashbin" you're talking about Spoiler: that arc where that arc where ML had the opportunity to be a sentient being around MC and he choose to be the AI of his mech?
i think it's Pleasing Start Over, arc 5 ML is an instructor of some sort and Mc is an Omega(I think), Ml keeps taking advantage of Mc while 'guiding' him and 'helping' him become stronger. wasn't that pleasant