http://www.mangago.me/read-manga/yuuutsu_na_asa/mf/v01/c004/28/
It was kinda obvious that Katsuragi wasn't ok with it. He did put some resistance, it left bruises. Akihito did force him even with Katsuragi physical resistance. Even if at first he was like "i don't care", kept his mouth shut and later lost his will to fight. It was clear that there was resistance(physical), it was rape.
Oh you're right. I read this months ago so I only remembered the general story. Oh gosh...now that I've revisted this chapter...all the feelings are surging back into me. It's a really sad chapter.
I don't get why the Rape tag is here. I thought the sex was more on dubious consent than actual forced coercion.