Well, when Gregor asks if something was going on between him & Kay, Marc says yes, which earns him a beating. That's a pretty big step from punching Kay and denying everything.
And what if it was only Kay? Do you believe that could be true? I have no frame of reference for that; I've only heard opinions that you can't be "gay" for 1 person........like Ennis was for Jack
Rewatch the scene, Marc never said yes, Gregor implied... What is Marc was just defending Kay's rights?
I'm not qualified to answer but I certainly understand how it could happen. Just recently Chris Martin has made allusions for many years and one most recently. Most realize when they encounter a really attractive person, they just know it, how else do we know the person is attractive? The heart and mind often do not agree and that was Marc's case. We realize and honor the fact that human sexuality happens on a ranged scale. The available range is influenced by social stigma, fear of death, beatings, bullying, supportive or unsupportive communities, laws, constitutional amendments or as we saw here, just approval from friends, parents, co-workers, ...
Bottom line: It is possible once one allows it to be. Cheers, V.