The “Surprise, They’re dating” meme takes an apparent dichotomy between two supposed opposites juxtapositioned against each other and says “Surprise! They’re dating!”
The example above shows the original, in which one formally clothed girl is juxtapositioned as “more attractive” than the loosely dressed girl. The intent behind this imposed dichotomy is to reinforce the conservative dress code as “superior” and make the informal one “inferior” in comparison.
What this amounts to is the creation of a patriarchal dichotomy represented through these archetypes. Where the Traditional “Trad Wife” is more “attractive” because of her apparent chastity. The informally dressed girl is “inferior” in contrast to this. The “subtle” message here is to reinforce slut-shaming and establish a socio-monoculture of relations for women to follow.
Basically, “Women should only dress in this one way in accordance with rigid gender roles.” This removes agency or at least tries to through spreading this message. The meme is created to reinforce a mono-relation.
What’s great about making this supposed dichotomy a couple is that you take an anti-individualist patriarchal meme and reinject individuality into it. You queer the juxtaposition, both metaphorically and literally. Now both girls, originally intended as rivals are now a couple.
The memetic mutation goes from mono to plural by doing this. Both girls are no longer seen as juxtaposed archetypes where one is presented as inherently better to the other, where only one, mono, is desirable. Being put as a couple means they’re both put into equal positions. Still retaining their individual differences too. The message now is “wear whatever and also love each other.”
Removing that negative and supposed superiority between them allows for the meme to go from reinforcing one relation to many, plural.