It's almost as though he has minimal expectations, not just of love but of everything, and settles for limitations in all sorts of ways. He also doesn't expect to be loved and can't really handle the idea, because he's never been seen as lovable till he meets Jack. (I 'm excluding Alma - that's just Ennis doing the conventional thing.) I can see him thinking of Jack as like some wonderful present, something you never dreamed of having, something you don't deserve, and so to be put away carefully and played with only occasionally. There's also the sense that he doesn't deserve Jack, and if he draws the attention of the gods they'll snatch him away.
AS far as love between two men ( or two women) is concerned, it's always existed. What varies is the amount of energy put by conventional society into repressing it and demonising those involved. Though it wasn't called that, there was certainly a gay scene in the UK in the sixties - I lived then in a smallish town on the borders of Wales where a friend was gay and out; he was part of a gay group who were discreet but not secretive. I think Ennis knew about the possibilities but just couldn't acknowledge where his inclinations lay, and so constantly refused to do anything other than shut the door.