In Japanese I believe there are 3 different ways to say "I love/like you."
There's the platonic/friend/family way, the "I have a crush on you" way and the more serious "I love you" between married couples or those who are serious about their feelings.
Just 'cause Oogami can recite a simple "I like you" in a setting doesn't mean his "I love you" for Hiroto is not genuine. That'd be like a movie star acting out the role of a loving spouse and saying "I love you" to their acting partner then going home and telling their actual significant other that they love them. One is just a line in a script and acted out while the other comes from a deeper place of meaning.
That and watching porn while gettin' freaky is more common than most of you people think-- our seme just so happens to be the one in the porno this time around. He wasn't tryin' to do it as some sort of weird flex but was doin' it as a way to put Hiroto and himself in the mood. That and he was tellin' Hiroto to focus on him in the end and didn't allow him to look away-- which was somethin' Hiroto was doin' out of feelin' insecure despite Oogami reassuring him previously that he was interested in him in both the physical and emotional aspect.
Yeah, i'm not saying that is wrong for him to be a sex worker or to watch porn while having sex, but even porn stars who enjoy that kind of thing know is better to keep their work separated from their personal life. It's also very weird to me that he didn't asked him first if that was okay for him to do that. It's not that he just knows it is okay, because that was just their second time together.
I also feel like is necessary to take into account that they're barely know each other, it's been very little time since they're reconnected, and saying "I love you" to a person who you don't really know, specifically because you know that that's what they want to hear doesn't sound very genuine to me.
Maybe I'm just projecting too much into it, but so far I'm very suspicious of this dude, I just can't help to sympathize better with Sakuma, since that's the perspective we're viewing, and I truly don't understand very well Oogami's motivation, so that's it for me
I don't know, I feel like the top is kinda sketchy. I'm guessing he had a big crush on him from highschool and he's acting on it, but still, the interest he's showing Sakuma doesn't feel genuine? Also, gotta be honest, it kinda sucked that he played his sex tape while having sex with Sakuma, he didn't even asked if that was okay with him, and Sakuma overall doesn't have a lot of confidence in himself, I felt like he was just rubbing salt on the wound. Specially because he showed that he can easily say "I love you" even if he doesn't mean it. I can already feel the angst coming