IDK.  I think it still means "women should base how they look on what men find attractive."  Yes, this man may be deviating from the norm a bit, but that doesn't make him any more enlightened, no matter how many women would like to think so.  We only have this man's word that his wife is exclusively dieting for him; I find that unbelievable.  I know so many women who diet.  The reasons are many: to be healthier, to look like models in magazines, to be more attractive to men in general, and so forth, and not once have I heard "to look good for one specific man."  Women are bombarded with the idea that being as thin as possible (not just thin) is how one looks their best.  While it's true that when men express a desire that deviates from the norm (I've heard a guy say he will not date a fat girl not because he has any particular aversion to heavy women, but because his fraternity brothers will make fun of him), I still don't like this card.  It strikes me as the man is completely unaware that his preferences aren't the only thing his wife might have in mind, and might not be the most important factor in her choice to loose weight.  

Also, I want to know if he's ever told her his preference.

This isn't helped by expectations of masculinity that are also foisted upon men, by both other men and women. "You're wife/fiancee/girlfriend is good looking!" is the expected response that a man is expected to hope to get from showing a photo of his wife/fiancee/girlfriend: a confirmation that the physical beauty of the woman with him matches the social expectation of beauty (or at least a beauty that matches -- or supersedes -- the socially expected physical beauty that a man of his type "deserves"). Showing off a photo of a woman who isn't "good-looking enough" for others' perception of him carries with it a social stigma for the man; and in this way any personal preference of the man is curtailed if he wishes to maintain this aspect of his social status. Conversely, a statement of surprise about a woman who is "too good-looking for him" can be seen as a slight to the man. It's almost a no-win situation, unless the woman fits with the perception of the man's social worth.

A man's social worth is determined -- to an extent -- by the types of things that he "has" (using "has" in a very loose sense here) or can claim credit for. These include some more-extrinsic things, such as the size of his bank account (and house, car engine, etc), his job standing, and the number and abilities of his children (and "zero children" carries with it a value judgment), as well as more-intrinsic things, such as his level of physical fitness (often normalized for his age), the amount, type and quality of his education, and his fashion sense ... and the "quality" of his wife/fiancee/girlfriend (fitting into the logic above). In some ways, we have probably all witnessed this, and many of us know how to make these characteristics fungible and -- therefore -- equatable:

"He's really fit and really handsome, why can't he find a better-looking girlfriend?", "Why is a good-looking woman interested in that fat schlub? He must be loaded," or "He's such a successful guy: he's rich, has a big house, two smart and talented kids, and a really beautiful wife."

On the other hand it's rare that we hear, "He's so lucky that his wife is a little heavy."

NOTE: This is based on what I've witnessed and experienced, and my experiences may not line up with others'.

That is an interesting take, especially if you consider how the Young 'uns mold themselves to fit the generally accepted type of attractiveness precisely because they don't know about these variations in desire. I think this also says a lot about biphobia: people are so used to attraction being all about a heteronormative gender binary that they can't wrap their heads around the possibility of forming your desires around other criteria

By the way, I know that it breaks expectations, but I've heard the "because I want you to look like *this* argument in regards to body type before, and it's really disempowering. The image unsettles me for that reason

One thing struck me: What evidence do we have that the author of the card is male?

This one really speaks to me. I'm a woman, and I know that my boyfriend prefers me a little bit on the chubby side. He hasn't said it explicitly, but if I ever accidentally lose a little weight, he frets and encourages me to eat more. If I joke about losing weight, he looks a little alarmed and tells me I don't need to. He definitely encourages me not to hold back with food! (For the record, I'm not too thin by any definition - BMI around 21 these days, I guess - but I'm short, so I only have to lose ~10 pounds to start to tip into the "too thin" zone.)

Anyway, it's so interesting how hard of a time I have believing that he truly likes my body as it is, and even maybe wishes it were a few pounds heavier. Clearly he's been more explicit about it than the sender of this card and I still have trouble believing it. I am accepting it, though, and I find it extremely comforting. It's really cool to just let my body be the weight it wants to be and not fret about losing weight. I'm desirable at this weight, so why should I change it? I won't deliberately gain weight for him, because I don't feel that that would be healthy, but I will choose to not lose weight for him!

I'm sorry, but this is purely racial. The fact that the "Mainstream Beauty" is a skinny woman is a bit condescending because now a days Blacks and Latin@s are as much part of the mainstream as Whites (even if not admittedly so) and not all have the same view of beauty as Caucasian people.

I'm attracted to natural women. I can appreciate fat women, or thin women, or those lovely plump people somewhere in the middle, but what I find most attractive about them, is the fact that they aren't putting on a facade. Unless it's for theatrical purposes, in which case it can become attractive again, simply because we're in on it.

Women who put on makeup, or struggle to maintain a set weight, or where high heels because it is part of their everyday facade, it's a turnoff.

Every time my mom goes on a diet, my step-dad starts buying her ice cream.