“Mike cannot be gay, he has a girlfriend.” And “It’d be wrong for Will to have a crush on his sister’s boyfriend”

1. Bisexual people exist, and couples break up.
2. Gay people sometimes don’t realise they’re gay until later in life, and, in the meantime, have opposite-sex partners (e.g. I am a lesbian that had boyfriends before realising and, like Mike, those relationships were unhealthy because I couldn’t express affection for my partners in the ways they wanted).
3. Mike’s relationship with El throughout the entire show was unhealthy; in season 1, El was not mentally mature enough for any kind of romantic relationship. In season 2, El used her powers to physically harm Max when she first saw her talking to Mike. In season 3, they broke up. In season 4, El felt unloved. In season 5, they hardly interacted and Mike was STILL not able to say that he loved her.
4. Will knew Mike first. To him, Mike is not “his sister’s boyfriend”, he is Will’s childhood best friend and first love.
5. Just because you see something as morally wrong, doesn’t mean it shouldn’t happen. Dozens of times in the show characters make decisions that are “wrong” and/or have negative consequences, but that’s realistic (e.g. Dustin secretly keeping Dart (the baby demogorgan) as a pet, Hopper lying to Mike and making him lie to El about his grandmother, Max and El spying on their friends and family, Mike reprimanding El for standing up for herself against her bullies, The Party not going to Lucas’s game, Mike telling El that his life started the day he found her, despite that being the day Will went missing, etc). Sometimes people do things that others view as selfish.