Well, I agree with Crayons in the general interpretation that Old Man Twist was an all-around unpleasant, rude and mean-spirited guy but not necessarily homophobic. If we extrapolate into the movie from the story we know he was abusive to Jack, but not necessarily based on Jack's sexuality especially since OMT's tendency towards abusiveness started when Jack was so young.
I think the fact that Jack told his parents about at least two men (Ennis and the "other fellow") in his life says a lot about how that aspect of his life was perceived and dealt with in his parents' house. And, I also think that OMT would have accepted Ennis on the ranch with Jack, as others have said, with the main objective from OMT's point of view being the care and maintenance of the ranch.
And, one point I think might be interesting to consider is that Ennis's encounter with OMT is part of his growing and learning experience following Jack's death. In meeting both of Jack's parents he learns that people could surprise him for the better in the nature of their reactions to their knowledge about his relationship with Jack. Ennis had gone through life expecting attacks and hatred at every turn when it came to people suspecting about his sexuality. But, upon meeting Jack's Mom (who clearly knows about his love for Jack) he encounters kindness and sympathy and someone who shares his grief. And, with OMT, it is a bit more complex because the conversation with him is so uncomfortable. OMT clearly is trying to needle Ennis about what he knows of Jack's statements about the other man, etc. But, still, OMT is not attacking Ennis or Jack with homophobic slurs or exhibiting the overt and classic signs of homophobia that Ennis might have expected.
