
I dont really know the specifics of labels but sei had consensual sex multiple times with misumi so that should already make him bi or gay, right?.. It just irks me whenever there's a "had sex with him but he's straight" in mangas
anyway sei-san should just confess im dyingg . dont want misumi to hurt

It's not just in mangas but irl too. Being straight is not exclusive to not having sex with gays. Being straight is that a person is emotionally attached and attracted to their opposite sex. They can have a romantic and committed relationship with their partner. Thus they can still have sex with the same gender yet still call themselves straight because there is no emotional attachment involved with their sex. That's pretty much what sexual orientation is about apart from being able to have sex to the respective gender. Most of the time, sex is just pleasure.
Though, there is a case I encountered where a guy who is straight (generally is sexually attracted to females) but is in a serious relationship with a gay or transgender. He is still attracted to women, when seeing them online or whatever but he is committed and sexually active with his gay partner. When they answer questions about their relationship, they don't like to label and they just say they are in love yet the guy is still labelled as straight.
Bottomline, according to my mere understanding, there are two part of attraction: emotional and sexual attraction. They don't always match and it would always depend on the person how they would label themselves. They may label themselves gay depending on who their serious partner is (to whom they are emotionally attracted to) or on who they are sexually attracted to (like how beautiful or sexy they are). Sorry it got long. It's hard to explain. If I wasn't clear since I am no expert, google gender and sexuality. There are A LOT of readings you would find.

Yeah pretty much. The second scenario I mentioned in mangas usually would be "I am not gay but I only feel this with you" kind of scenario or line, or kind of like "I still like girls or is attracted to them but I love you and only want to be with you", pretty much what seme's say when uke's are extremely insecure or friends turned lovers plots. I've read those before and I know a couple irl who are like that. If you are familiar with Jeffree Star on YouTube, his relationship with his boyfriend is exactly what I made example of.
It's complicated really but after reading a lot I kind was able to map out how these things happen. Lol. But what I sort of understand is that your label, straight/gay/lesbian, is based on what gender you are attracted to e.g. you see a picture of a hot guy, for straight males/lesbian: they don't find them hot or turned on; straight female/gay: they find them hot or turned on BUT their respective romantic partners may be or not the gender they randomly find hot, e.g. straight male but his rom. partner is gay, s-male finds his partner hot or beautiful (since they love them and are emotionally attached and attracted to them) but given other gay men presented to them, they don't feel anything or not find them hot.
What a long explanation for what a gender label is! Whahha. Though I am not sure if this is indeed how it is defined since there are asexuals, aromantics, pansexual, etc. Sorry I just repeated myself. Anyway, happy to sort of help.
this whole series is shit but why am I still reading this
Same thing I ask myself. I hate everyone in this series. The only passable character is the damn bartender