I think the idea that Monroe serves as a foil is very good.
He's a classic nice guy it seems to me. I think his image as less-than-macho or perhaps even a "sissy" is everything to do with the underlying stereotypes that have to do with sexuality (maybe more so... or linked with stereotypes about masculinity). In this case, the reliable straight husband is the "sissy" and the gay man is the macho, tough guy. Making the contrast between Ennis and Monroe marked and very obvious I think was certainly intentional in this regard.
IMO it might have been to mix it up a bit, especially for viewers who are moving outside their comfort zone in terms of how they see Ennis and Jack. i.e., that late in the movie, the viewer imight be comfortable with considering the gay character masculine but just as you thought it was safe to go back in the water, here comes another reversal.
Monroe is the classic nice guy who finishes last but also could be many a woman's good catch as a second husband. In high school and early adulthood, he'd be the nerd or the dull workaholic that most females would overlook in favor of the guy who's more macho, better-looking , maybe with more of an aura of danger about him. Unfortunately, bad-boys don't age nearly as well as nice-guys and tend more than the Monroes of the world to be lead characters in very bad marriages -- some much worse than Alma's that's for sure. The Monroes more often improve with age.
And, I don't think we're too far off topic. The electric knife is also all about stereotypes and even sexual stereotypes (metaphorically speaking).
Well, that can certainly put the Swords suit in the Tarot in a whole new light!
