I think sensei was a beta? Am I wrong?
You know what? I don't know, you may be right. I had to read the whole thing again to verify, because I couldn't remember ever reading it in the manga, and I couldn't find any reference to his gender. It's never mentioned whether he's an alpha, beta or omega. If he's is a beta it would explain why he could keep calm during sex with Yuu and Tomo (Tomo was in his heat). I guess I assumed because of his dominant personality and because of the tags that he was an alpha. Also there is a description of the manga which says: "An alpha and omega are happy together as lovers... till an alpha guy gets interested in one of them.". But that is a modified version from the translators summary which says: "Two omega boys are happy together as childhood friends, but are really in love with each other. Things are relatively peaceful… till an alpha guy gets interested in one of them."
So my takeaway from this is, you may be right, I don't know. I don't think it affects much of my previous comment though.
Sensei did say something about having sex on a heat, but he didn't go crazy, so maybe he's a Beta... Even Betas can feel an Omega's pheromones!
What you say makes sense, I wish we could get a definite answer though. I wonder if in the Japanese version it's implied in some kanji but doesn't translate well to English or if it's never established either.
They bonded, recognizing each other in themselves. Sensei finally found someone who actually understood him. A true soul bond. Doesn't excuse the fact that Sensei was a total creeper but it does (I think ) soften the 'blow' (so to speak) of those two ending up together.
I wanted to express a hypothesis I have and see if anyone else thinks it may be possible. So, I get the feeling that when we get to see Sensei's childhood flashback, he is kissing or doing something sexual to another kid in the orphanage ( http://www.mangago.zone/read-manga/sukijanaitte_hyakkai_tonaeta/mf/a7ee16e42adba2df7b841cbe75a73dab_chapter-6/pg-26/) because on the next page the caretaker says "what a dirty kid". My hypothesis then is that maybe he was sexually abused as kid and that is related to why he got sent to an orphanage even though he was an alpha. I say this because it seems to be common for children victims of sexual abuse to be hypersexual (I'm not sure if that is the correct term, but they tend to repeat the abuse on other children without knowing that what they are doing isn't normal).
If this is the case it could partially account for why he is such a fucked-up person (not that it necessarily justifies it). I also get the notion that by the end of the story the mangaka is trying to get the audience to sympathize with Sensei, like when he shows that sort of sad or "nostalgic" face (here: http://www.mangago.zone/read-manga/sukijanaitte_hyakkai_tonaeta/mf/a7ee16e42adba2df7b841cbe75a73dab_chapter-5/pg-25/ and here: http://www.mangago.zone/read-manga/sukijanaitte_hyakkai_tonaeta/mf/a7ee16e42adba2df7b841cbe75a73dab_chapter-5/pg-26/) when seeing another couple being truly happy. I think my hypothesis might go along with the mangaka trying to make as see him in a more favorable light.
Ps: When I say Sensei I mean the character and when I say mangaka I mean the mangaka (obviously XD). Oh, and sorry about the horribly long links, I don't know to make them shorter or a direct link.