"Eh! It's rare to see you two fight!!"
That MC-kun even told the most aggressive and unrepentant one of the three who caused the fight as if she wasn't involved is confounding.
"Wait, wait! Don't pull me so hard!"
Not so much an instance of his stupidity as it is his effeminacy; I guess his ending up in drag makes sense. But what is of his stupidity is his demanding to know what the girl is planning to do--which is an example of his incredible passivity: he even had the audacity to mouth off to Yukino about being "selfish" when he was indeed engaging in infidelity for exactly the accusation Yukino leveled against him.
"How am I supposed to stay calm when you brought me to a place like this...?"
So he didn't choose to comply in putting on the uniform and walking into that lingerie store, and then into that changing booth with her. Good to know; looked pretty different to me.
Given everything that girl has done so far--i.e., aggressive homewrecking with an eye toward feminizing him--she's obviously got her perverted fetish in mind if she's going to have him crossdress, and he still claims to himself that "Aya-nee is trying her best" to help him. She takes off her shirt, shows herself to him in the bra, and eventually has him grope her tiny tits directly.
Then comes the move the girl was so obviously going to make on him--and, like a deer caught in headlights, he was going to just let himself (and, thereby, his relationship with Yukino) get hit by that oncoming car...until someone else stopped him by chance.
"Aya-nee seemed a little different from usual."
Yes, don't remember her going so far as to bare her body in order to steal him from Yukino. Just a little different, though.
"N-no, no way! I'm devoted to Yuki-nee!"
This reads like this manga was an NTR all along, with the poor victim being Yukino--and it's already seemed that way long ago, with just how much this guy lets these girls have their way with him. Exactly how devoted could he be when he went as far as he did shortly before he said that? He went along with that girl and groped her chest without restraint after letting that girl put his hand on it (because, you know, adjusting a worn bra doesn't involve the breasts underneath), to name just one thing that proves how "devoted" he is.