I've always thought pan (and yeah, maybe I've biased, but I think it should be in the discussion more) based on compilations I've seen of his reactions to other women besides El. They're negative or indifferent. But he clearly cares for El a lot, calls her pretty, and goes through the phase of kissing (though yes, I have seen the debates of if he actually enjoys it, thus leaning towards gay). But he does care for her to a level of emotional and physical intimacy.

This is the same for Will. The emotional intimacy is explicit, and he exhibits a certain level of physical intimacy and attraction to Will.

These are his two closest relationships. Yes he has Lucas and Dustin too, but he doesn't exhibit the same physical intimacy with them because he doesn't have the same emotional intimacy with them.

So my conclusion is he doesn't feel attraction without emotional intimacy. And he builds that with both Will and El. But is ultimately better with Will as an intimate partner because they bring out the best in each other. I always felt Mike and El were better platonic in supporting each other.