Even though they were more like sex friends than lovers but ml keep saying that he likes him loves him so I think it is count as a cheating. When you like someone and approach them, having sex with them and continuesly saying I love you so I guess you are in relationship in some way. So it's definitely a cheating.
1st He (seme) had sex with that uke (B)when he wasn't in relationship with uke (A).
2nd seme keeps on saying he loves A, like a lot and he said to B that he likes him, because he taught him many things, he sees B as a friend. Both seme and B did felt guilty about that night. B said to seme when A saw them together in resturent, they were talking. Yes A didn't trust seme but It's a misunderstanding.
If you think it's cheating then it's your opinion, I'm sharing mine.
They weren't even boyfriends when he 'cheated'. They just were more like sex buddies..so it's not cheating. They became official right after