I have super specific request - are there any stories where female is forced to gain weight for/before marriage either cultural thing or her partner makes a condition for her to gain some amount of weight for them to marry?

I got one, but it’s far less “innocent” as the picture you presented.

A rather selfish Earth celebrity gets inducted into a galactic marriage, but it comes with the caveat that wives undergo similarly cosmic levels of growth to be considered a rightful spouse, to which she undergoes without hesitation.

https://www.deviantart.com/adipose-rex/art/Wedding-Preparations-1047157555

So this likely won’t scratch that itch if you want a meek partner undergoing a new lifestyle change to get married. But if you like *absurd* stature XWG stories for a wedding ceremony, you’ll probably like this too.

There used to be an old story on the Dimensions Weight Room, or maybe the message board, about a woman who was supposed to get married to some rich Balinese guy and was immensely fattened as a result. The ending got quite weird but the bulk of the story was very enjoyable. I forget the name of it. It was particularly hot because in addition to her ever-increasing bulk, the woman was depicted as changing in personality as she became fatter, actively becoming greedier and more gluttonous. Not all stories get that part right.

Oooh! Got a perfect one for ya! 4 part sequence with story for each one below.

https://www.deviantart.com/w-oo-t/art/Fitting-In-pt1-536495503

It stars a very health focused girl marrying into a family, and her fiancé advises her that eating a hefty banquet at the yearly family cookout is key to fitting in.

Each year, we see how her attitude slowly shifts to being more gluttonous, and her figure follows suit. This *absolutely* sounds like the vibe you were looking for!