The way the majority of romantic Milkvan screentime for Mike is dedicated to social performance for the purpose of bonding with friends and not actually being romantic with her. He's happy platonic with her, unhappy romantically with her, or doing it for others.
Bonding with his sister for the first time in years.
Our girlfriends broke up with us let's hang out so you can show me the ropes.
Let's compare girlfriends as a conversation topic because I enjoy talking to you.
Will hasn't talked to me all day, I'll look super happy without him to try to get him to notice]
But for those first few is really my focus here. He is socially incentivized to have a girlfriend.
The Nancy one is HUGE, I just remembered while writing this post! They have not been close in years and one of the first moments they are again, they talk about crushes. I have done the exact same thing. Bonding is scarce and it's important to you now, the conversation topic is this, engage with it! Even if it's a teasy denial, it's a back and forth.
He does not call El for multiple days after the fight then breakup in season 3. But what does he do? Talk with and spend time with Lucas about it! He is bonding with Lucas over a shared interest at the end of the day. They go to the mall together, Lucas sleeps over, they make jokes together about the DnD game. All of this is bonding, but none of it is in much actual pursuit of El. It's also pertinent to notice that they were fighting over El not that long ago, him and Lucas. This is nice. To relate again. They didn't have peace with each other, then they didn't have peace from the world. They've got a little now and he's enjoying it. Enjoying this time to just spend time with and relate to his friend.
And in season 4, their friend group is strained right now.
El is a TOPIC for him to bond with others over, or at least as a "girlfriend" she is. It's why we see a lot of him talking about her with others positively without the actions to back it up. The talking was his goal. Let's go shop for apology gifts together. You're talking about girlfriends? I'll talk about girlfriends! Give me advice, that's bonding.
Not only that, but I noticed that all my examples also came up when he was in need of bonding. He spent season 1 fighting with Lucas and the time from season 1 through season 2 isolating himself or being with Will only. The Snow Ball on is when he's really back to him spending real quality time with Lucas. That's something to cherish. In season 1, he hadn't really talked to Nancy in years. This was the first thing she asked in ORDER TO BOND after EXPLICTLY SAYING THAT'S WHAT SHE WAS TRYING TO DO. And in season 4, their friend group is strained and separated. Max is isolating, Lucas is off with his other group. In many ways, it is just Mike and Dustin, and they're both stressed about this fracturing. It's nice to talk like this.
Talking about girlfriends feels normal, in more ways than one. Makes him feel normal, but also makes his life and relationships feel normal when they aren't. It's not just performative for an audience to be convinced, it's a social act.
He feels there's something he would lose. Similar to El when she kisses Mike before they move, familiarity is nice in these times. Big changes make you seek some feeling of normalcy.
So it's not just his defensive of his sexuality. He says in season 4 he does not feel as secure in his relationships and maybe we do on behalf of him. He was worried he was losing Lucas, Will, and El all in season 4, for example. And it has been long before that he had the insecurities he discusses that he is unworthy and everyone's gonna wake up to that one day.
He doesn't just lose his closet. He loses a sense of connection he has. Even though it would never really cost him his friendships if they thought he was just...straight and single. With all the changes in their lives, personalities, everything. "We're not kids anymore", he's feeling it too, with all of those changes, he also doesn't want to lose one more means of connection. He's protective of that too.
If he and Dustin are dancing around the subject of Lucas and The Byers being gone and everything that's wrong and they don't know how to fill that painful silence...they can talk about their girlfriends. He doesn't want to lose that either.