You think she told Monroe later? Before that Thanksgiving dinner? I wonder about this, will have to give it some thought, think about what kind of person Monroe is. He didn't seem the slightest bit uncomfortable with Ennis, which is interesting in itself -- he didn't see him as a threat at all. Which could mean he knew Ennis's inclinations were elsewhere. But if he knew this, what would his attitude have been? What kind of social perceptions did he have? Maybe he was a little ahead of his time, and maybe the grocery business put him in touch with all kinds of people and he might have had tolerance for people who were different. Although gay was not an acceptable "different." So, if he did know, would he be the kind that wouldn't want such a man around his step-daughters? He wouldn't really have anything to say about that, though. But once his and Alma's kids were born, would he want Ennis to stay away. Anyway, at the T'giving dinner, he was just mellow, and didn't over-react to Ennis's storming out of there, either.
Just thinking out loud.
kathy
I never thought I'd say this Kathy, but I see it totally differently

I thought Monroe seemed very uncomfortable in the Thanksgiving scene, almost like an outsider in his own home.He stops carving the turkey so Ennis can tell his story about the rodeo, then when Ennis is finished Monroe looks at Alma, as if to check that it's ok to carry on,I suppose it could just be that he was just being polite, but it felt to me like he didn't appreciate having to interrupt what he was doing for Ennis,and he would've appreciated Alma telling Jr that Ennis would tell the story once Monroe had finished carving the turkey but he didn't feel that he could say anything(actually, typing that just made me think that there's a similarity there with Jack's Thanksgiving scene with LD, maybe that's what put the idea in my head?

It never stops, does it?

)
I think Alma wore the trousers in the relationship with Monroe,and he would've been aware that wasn't the case with Alma and Ennis,even if Alma hadn't told him anything about her and Ennis, he would've realised that from the scene in the supermarket.Monroe strikes me as the sort of guy that is always bemoaning the fact that girls always like the bad guys and never the "nice" guys like him, so in that sense he might have felt inferior in some way to Ennis(assuming Alma
hadn't told him about Ennis and Jack, which personally I don't think she would). Not to mention the fact that physically Ennis was taller and more muscular than Monroe, I think Monroe would've found him very intimidating physically as well, that's the only reason I can think of for him not getting up off his chair when he heard all the commotion from the kitchen, he was very slow to react, and when he does he hardly reacts at all, Ennis, for all his faults as a husband, would've been in there like a shot if it had been the other way around.
You would think that afterwards Monroe would've asked Alma for an explanation, but I get the feeling that Alma would've said she didn't want to discuss it and Monroe would've just left it, he doesn't seem like the type to push the issue. I don't think Alma would've told Monroe about Ennis,she couldn't bring herself to confront Ennis about it for years, she was in denial about it herself for years, even went to the trouble of setting the "trap" with the fishing line, rather than just telling him what she'd seen, and then when she
does confront him, it's with the creel case story, if she really wanted to force him to admit it, why not just say that she's seeing him kissing Jack? there's no way he could've said
that didn't prove anything

I think that even after the divorce, Alma didn't want to believe that Ennis was in love with another man, maybe because of what she thought that might say about her, wasn't she a good enough wife, a "real" woman, if her husband preferred being with a man to being with her? She might have worried it would reflect badly on her if she told Monroe, and didn't want to put her new marriage at risk, either way, I don't see that she had anything to gain from telling him.
Blimey, I can't believe I wrote such a long rambling post about Monroe, of all people! :-[ Jack and/or Ennis is one thing, but Monroe?!