I think that Alma was in love with Ennis, but I don't think that he was in love with her. I think Ennis may have loved her as a person, but that is it. I think he married her probably because he though she would make a good wife and she probably showed a romantic interest in him and to remain single might have aroused suspicion and could have made Ennis feel paranoid. We don't know if he ever felt romantic and/or sexual attraction to another man before meeting Jack, I kind of doubt that he did. Did Jack ever have a romantic and/or sexual interest in another man before meeting Ennis, we don't know. Even though Jack and Ennis are both the same gender, they both have very different personalities, they are almost like the exact opposites of each other and a romantic and sexual relationship developing between them seems almost highly unlikely if you think about it.
Alma wanted her marriage to Ennis be more satisfying, but it did not turn out that way. I can imagine her thinking that she was going to have a very happy life with him when they were engaged and probably dreamed of that when they were dating. Ennis grew up in poverty and wasn't very well-educating, so it would have been a challenge for him to provide financially for Alma and their two daughters. Jack, he married a pretty young woman from a well-to-do family. No doubt, Jack grew up in poverty and to marry into a family and find a job that helped him move up the economic ladder was a welcome change I imagine for him.
I think he probably thought Lureen seemed nice , was pretty and would make a decent wife. Lureen is a university graduate and seems to have been a talented rodeo queen. She joined her father's business and seemed to throw herself into her work. Her father cannot stand Jack, so leaving his marriage with Lureen probably would have had an upside by getting away from L.D., although what future prospects for a good job would he have had if he left Lureen? If Jack studied to get his G.E.D. and then went to college, then his chances might have been better for having a good career if he decided to leave Lureen. He also had to think about Bobby, I'm sure Jack loved his son, but it was probably hard for him to be the kind of father that he must have felt Bobby deserved, especially when Jack didn't seem to have such a loving father who seemed to look at his son as a disappointment to a degree, little more than a silly dreamer with his head in the clouds. Ennis, Jack, Alma and Lureen are all pretty much victims of their circumstances and none of them get what they really want. I think that is one of the things that makes the film so sad.